feat(library): add secondary "Then by" sort with smart defaults (#4347)

Adds a primary/secondary sort pair so users can group by author and have
each author's books drilled-in list sort by series, without touching the
sort menu each time. Closes #4307.

- New "Then by..." picker in the library view menu (None + same keys as
  primary). Secondary acts as tiebreaker for the global sort, and as the
  in-group ordering when the user drills into a non-series group.
- Smart defaults derived from groupBy, surfaced as "(Auto)" in the menu
  and resolved at sort time so user picks are never overwritten:
  - groupBy=Author + secondary=none -> Series
  - groupBy=Series + librarySortByAuto -> primary becomes Series
- librarySortByAuto flips off as soon as the user makes any explicit
  primary pick; subsequent groupBy changes then respect that choice.

Settings: librarySortBy2, librarySortByAuto. URL: ?sort2 syncs the
secondary; auto is settings-only (no URL representation).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/readest-app/public/locales/ar/translation.json

@@ -1778,5 +1778,6 @@
   "No book content or reading data": "بدون محتوى كتب أو بيانات قراءة",
   "Share anonymous data": "مشاركة بيانات مجهولة",
   "Not now": "ليس الآن",
-  "You can change this anytime in Settings.": "يمكنك تغيير هذا في أي وقت من الإعدادات."
+  "You can change this anytime in Settings.": "يمكنك تغيير هذا في أي وقت من الإعدادات.",
+  "Then by...": "ثم حسب..."
 }

apps/readest-app/public/locales/bn/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "কোনো বইয়ের বিষয়বস্তু বা পড়ার ডেটা নেই",
   "Share anonymous data": "বেনামি ডেটা শেয়ার করুন",
   "Not now": "এখন নয়",
-  "You can change this anytime in Settings.": "আপনি যেকোনো সময় সেটিংসে এটি পরিবর্তন করতে পারেন।"
+  "You can change this anytime in Settings.": "আপনি যেকোনো সময় সেটিংসে এটি পরিবর্তন করতে পারেন।",
+  "Then by...": "তারপর..."
 }

apps/readest-app/public/locales/bo/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "དཔེ་ཆའི་ནང་དོན་ནམ་ཀློག་པའི་གནས་ཚུལ་མེད།",
   "Share anonymous data": "མིང་མེད་གནས་ཚུལ་མཉམ་སྤྱོད།",
   "Not now": "ད་ལྟ་མིན།",
-  "You can change this anytime in Settings.": "ཁྱེད་ཀྱིས་འདི་སྒྲིག་འགོད་ནང་དུས་ནམ་ཡང་བཟོ་བཅོས་གནང་ཆོག།"
+  "You can change this anytime in Settings.": "ཁྱེད་ཀྱིས་འདི་སྒྲིག་འགོད་ནང་དུས་ནམ་ཡང་བཟོ་བཅོས་གནང་ཆོག།",
+  "Then by...": "དེ་ནས..."
 }

apps/readest-app/public/locales/de/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "Keine Buchinhalte oder Lesedaten",
   "Share anonymous data": "Anonyme Daten teilen",
   "Not now": "Nicht jetzt",
-  "You can change this anytime in Settings.": "Du kannst dies jederzeit in den Einstellungen ändern."
+  "You can change this anytime in Settings.": "Du kannst dies jederzeit in den Einstellungen ändern.",
+  "Then by...": "Dann nach..."
 }

apps/readest-app/public/locales/el/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "Κανένα περιεχόμενο βιβλίου ή δεδομένα ανάγνωσης",
   "Share anonymous data": "Κοινοποίηση ανώνυμων δεδομένων",
   "Not now": "Όχι τώρα",
-  "You can change this anytime in Settings.": "Μπορείτε να το αλλάξετε ανά πάσα στιγμή από τις Ρυθμίσεις."
+  "You can change this anytime in Settings.": "Μπορείτε να το αλλάξετε ανά πάσα στιγμή από τις Ρυθμίσεις.",
+  "Then by...": "Στη συνέχεια κατά..."
 }

apps/readest-app/public/locales/es/translation.json

@@ -1685,5 +1685,6 @@
   "No book content or reading data": "Sin contenido de libros ni datos de lectura",
   "Share anonymous data": "Compartir datos anónimos",
   "Not now": "Ahora no",
-  "You can change this anytime in Settings.": "Puedes cambiar esto en cualquier momento en Ajustes."
+  "You can change this anytime in Settings.": "Puedes cambiar esto en cualquier momento en Ajustes.",
+  "Then by...": "Después por..."
 }

apps/readest-app/public/locales/fa/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "بدون محتوای کتاب یا داده‌های مطالعه",
   "Share anonymous data": "اشتراک داده ناشناس",
   "Not now": "اکنون نه",
-  "You can change this anytime in Settings.": "می‌توانید این تنظیم را هر زمان از تنظیمات تغییر دهید."
+  "You can change this anytime in Settings.": "می‌توانید این تنظیم را هر زمان از تنظیمات تغییر دهید.",
+  "Then by...": "سپس بر اساس..."
 }

apps/readest-app/public/locales/fr/translation.json

@@ -1685,5 +1685,6 @@
   "No book content or reading data": "Aucun contenu de livre ni donnée de lecture",
   "Share anonymous data": "Partager les données anonymes",
   "Not now": "Pas maintenant",
-  "You can change this anytime in Settings.": "Vous pouvez modifier ce choix à tout moment dans les Paramètres."
+  "You can change this anytime in Settings.": "Vous pouvez modifier ce choix à tout moment dans les Paramètres.",
+  "Then by...": "Puis par..."
 }

apps/readest-app/public/locales/he/translation.json

@@ -1685,5 +1685,6 @@
   "No book content or reading data": "ללא תוכן ספרים או נתוני קריאה",
   "Share anonymous data": "שתף נתונים אנונימיים",
   "Not now": "לא עכשיו",
-  "You can change this anytime in Settings.": "אפשר לשנות זאת בכל עת בהגדרות."
+  "You can change this anytime in Settings.": "אפשר לשנות זאת בכל עת בהגדרות.",
+  "Then by...": "ואז לפי..."
 }

apps/readest-app/public/locales/hi/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "कोई पुस्तक सामग्री या पठन डेटा नहीं",
   "Share anonymous data": "गुमनाम डेटा साझा करें",
   "Not now": "अभी नहीं",
-  "You can change this anytime in Settings.": "आप इसे कभी भी सेटिंग्स में बदल सकते हैं।"
+  "You can change this anytime in Settings.": "आप इसे कभी भी सेटिंग्स में बदल सकते हैं।",
+  "Then by...": "फिर इसके अनुसार..."
 }

apps/readest-app/public/locales/hu/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "Nincs könyvtartalom vagy olvasási adat",
   "Share anonymous data": "Anonim adatok megosztása",
   "Not now": "Most nem",
-  "You can change this anytime in Settings.": "Ezt bármikor megváltoztathatod a Beállításokban."
+  "You can change this anytime in Settings.": "Ezt bármikor megváltoztathatod a Beállításokban.",
+  "Then by...": "Másodlagosan..."
 }

apps/readest-app/public/locales/id/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "Tidak ada konten buku atau data baca",
   "Share anonymous data": "Bagikan data anonim",
   "Not now": "Nanti saja",
-  "You can change this anytime in Settings.": "Anda dapat mengubahnya kapan saja di Pengaturan."
+  "You can change this anytime in Settings.": "Anda dapat mengubahnya kapan saja di Pengaturan.",
+  "Then by...": "Lalu berdasarkan..."
 }

apps/readest-app/public/locales/it/translation.json

@@ -1685,5 +1685,6 @@
   "No book content or reading data": "Nessun contenuto dei libri o dato di lettura",
   "Share anonymous data": "Condividi dati anonimi",
   "Not now": "Non ora",
-  "You can change this anytime in Settings.": "Puoi modificarlo in qualsiasi momento dalle Impostazioni."
+  "You can change this anytime in Settings.": "Puoi modificarlo in qualsiasi momento dalle Impostazioni.",
+  "Then by...": "Poi per..."
 }

apps/readest-app/public/locales/ja/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "書籍の内容や読書データは含まれません",
   "Share anonymous data": "匿名データを共有",
   "Not now": "後で",
-  "You can change this anytime in Settings.": "この設定はいつでも設定画面から変更できます。"
+  "You can change this anytime in Settings.": "この設定はいつでも設定画面から変更できます。",
+  "Then by...": "次に並び替え..."
 }

apps/readest-app/public/locales/ko/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "도서 내용이나 독서 데이터 없음",
   "Share anonymous data": "익명 데이터 공유",
   "Not now": "나중에",
-  "You can change this anytime in Settings.": "설정에서 언제든지 변경할 수 있습니다."
+  "You can change this anytime in Settings.": "설정에서 언제든지 변경할 수 있습니다.",
+  "Then by...": "다음 기준..."
 }

apps/readest-app/public/locales/ms/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "Tiada kandungan buku atau data bacaan",
   "Share anonymous data": "Kongsi data tanpa nama",
   "Not now": "Bukan sekarang",
-  "You can change this anytime in Settings.": "Anda boleh menukar ini pada bila-bila masa dalam Tetapan."
+  "You can change this anytime in Settings.": "Anda boleh menukar ini pada bila-bila masa dalam Tetapan.",
+  "Then by...": "Kemudian mengikut..."
 }

apps/readest-app/public/locales/nl/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "Geen boekinhoud of leesgegevens",
   "Share anonymous data": "Anonieme gegevens delen",
   "Not now": "Niet nu",
-  "You can change this anytime in Settings.": "Je kunt dit op elk moment wijzigen in Instellingen."
+  "You can change this anytime in Settings.": "Je kunt dit op elk moment wijzigen in Instellingen.",
+  "Then by...": "Vervolgens op..."
 }

apps/readest-app/public/locales/pl/translation.json

@@ -1716,5 +1716,6 @@
   "No book content or reading data": "Bez treści książek i danych o czytaniu",
   "Share anonymous data": "Udostępnij anonimowe dane",
   "Not now": "Nie teraz",
-  "You can change this anytime in Settings.": "Możesz to zmienić w dowolnej chwili w Ustawieniach."
+  "You can change this anytime in Settings.": "Możesz to zmienić w dowolnej chwili w Ustawieniach.",
+  "Then by...": "Następnie..."
 }

apps/readest-app/public/locales/pt-BR/translation.json

@@ -1685,5 +1685,6 @@
   "No book content or reading data": "Sem conteúdo de livros ou dados de leitura",
   "Share anonymous data": "Compartilhar dados anônimos",
   "Not now": "Agora não",
-  "You can change this anytime in Settings.": "Você pode alterar isso a qualquer momento em Configurações."
+  "You can change this anytime in Settings.": "Você pode alterar isso a qualquer momento em Configurações.",
+  "Then by...": "Depois por..."
 }

apps/readest-app/public/locales/pt/translation.json

@@ -1685,5 +1685,6 @@
   "No book content or reading data": "Sem conteúdo de livros ou dados de leitura",
   "Share anonymous data": "Partilhar dados anónimos",
   "Not now": "Agora não",
-  "You can change this anytime in Settings.": "Pode alterar isto a qualquer momento nas Definições."
+  "You can change this anytime in Settings.": "Pode alterar isto a qualquer momento nas Definições.",
+  "Then by...": "Depois por..."
 }

apps/readest-app/public/locales/ro/translation.json

@@ -1685,5 +1685,6 @@
   "No book content or reading data": "Fără conținut din cărți sau date despre citire",
   "Share anonymous data": "Partajează date anonime",
   "Not now": "Nu acum",
-  "You can change this anytime in Settings.": "Poți modifica această opțiune oricând din Setări."
+  "You can change this anytime in Settings.": "Poți modifica această opțiune oricând din Setări.",
+  "Then by...": "Apoi după..."
 }

apps/readest-app/public/locales/ru/translation.json

@@ -1716,5 +1716,6 @@
   "No book content or reading data": "Без содержимого книг и данных о чтении",
   "Share anonymous data": "Поделиться анонимными данными",
   "Not now": "Не сейчас",
-  "You can change this anytime in Settings.": "Это можно изменить в любое время в Настройках."
+  "You can change this anytime in Settings.": "Это можно изменить в любое время в Настройках.",
+  "Then by...": "Затем по..."
 }

apps/readest-app/public/locales/si/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "පොත් අන්තර්ගතය හෝ කියවීමේ දත්ත නැත",
   "Share anonymous data": "අනාමික දත්ත බෙදාගන්න",
   "Not now": "දැන් නොවේ",
-  "You can change this anytime in Settings.": "ඔබට මෙය ඕනෑම විටක සැකසුම් තුළ වෙනස් කළ හැකිය."
+  "You can change this anytime in Settings.": "ඔබට මෙය ඕනෑම විටක සැකසුම් තුළ වෙනස් කළ හැකිය.",
+  "Then by...": "ඉන්පසු..."
 }

apps/readest-app/public/locales/sl/translation.json

@@ -1716,5 +1716,6 @@
   "No book content or reading data": "Brez vsebine knjig ali bralnih podatkov",
   "Share anonymous data": "Deli anonimne podatke",
   "Not now": "Ne zdaj",
-  "You can change this anytime in Settings.": "To lahko kadar koli spremenite v Nastavitvah."
+  "You can change this anytime in Settings.": "To lahko kadar koli spremenite v Nastavitvah.",
+  "Then by...": "Nato po..."
 }

apps/readest-app/public/locales/sv/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "Inget bokinnehåll eller läsdata",
   "Share anonymous data": "Dela anonyma data",
   "Not now": "Inte nu",
-  "You can change this anytime in Settings.": "Du kan ändra detta när som helst i Inställningar."
+  "You can change this anytime in Settings.": "Du kan ändra detta när som helst i Inställningar.",
+  "Then by...": "Sedan efter..."
 }

apps/readest-app/public/locales/ta/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "புத்தக உள்ளடக்கம் அல்லது வாசிப்புத் தரவு இல்லை",
   "Share anonymous data": "அநாமதேய தரவைப் பகிர்",
   "Not now": "இப்போது வேண்டாம்",
-  "You can change this anytime in Settings.": "அமைப்புகளில் எப்போது வேண்டுமானாலும் இதை மாற்றலாம்."
+  "You can change this anytime in Settings.": "அமைப்புகளில் எப்போது வேண்டுமானாலும் இதை மாற்றலாம்.",
+  "Then by...": "பிறகு..."
 }

apps/readest-app/public/locales/th/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "ไม่มีเนื้อหาหนังสือหรือข้อมูลการอ่าน",
   "Share anonymous data": "แชร์ข้อมูลแบบไม่ระบุตัวตน",
   "Not now": "ไม่ใช่ตอนนี้",
-  "You can change this anytime in Settings.": "คุณสามารถเปลี่ยนแปลงสิ่งนี้ได้ทุกเมื่อในการตั้งค่า"
+  "You can change this anytime in Settings.": "คุณสามารถเปลี่ยนแปลงสิ่งนี้ได้ทุกเมื่อในการตั้งค่า",
+  "Then by...": "จากนั้น..."
 }

apps/readest-app/public/locales/tr/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "Kitap içeriği veya okuma verisi yok",
   "Share anonymous data": "Anonim veri paylaş",
   "Not now": "Şimdi değil",
-  "You can change this anytime in Settings.": "Bunu istediğiniz zaman Ayarlar'dan değiştirebilirsiniz."
+  "You can change this anytime in Settings.": "Bunu istediğiniz zaman Ayarlar'dan değiştirebilirsiniz.",
+  "Then by...": "Sonra..."
 }

apps/readest-app/public/locales/uk/translation.json

@@ -1716,5 +1716,6 @@
   "No book content or reading data": "Жодного вмісту книг чи даних читання",
   "Share anonymous data": "Поділитися анонімними даними",
   "Not now": "Не зараз",
-  "You can change this anytime in Settings.": "Ви можете змінити це будь-коли в Налаштуваннях."
+  "You can change this anytime in Settings.": "Ви можете змінити це будь-коли в Налаштуваннях.",
+  "Then by...": "Потім за..."
 }

apps/readest-app/public/locales/uz/translation.json

@@ -1654,5 +1654,6 @@
   "No book content or reading data": "Kitob mazmuni yoki oʻqish maʼlumotlari yoʻq",
   "Share anonymous data": "Anonim maʼlumotlarni ulashish",
   "Not now": "Hozir emas",
-  "You can change this anytime in Settings.": "Buni xohlagan vaqtda Sozlamalarda oʻzgartirishingiz mumkin."
+  "You can change this anytime in Settings.": "Buni xohlagan vaqtda Sozlamalarda oʻzgartirishingiz mumkin.",
+  "Then by...": "Keyin..."
 }

apps/readest-app/public/locales/vi/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "Không có nội dung sách hoặc dữ liệu đọc",
   "Share anonymous data": "Chia sẻ dữ liệu ẩn danh",
   "Not now": "Để sau",
-  "You can change this anytime in Settings.": "Bạn có thể thay đổi tùy chọn này bất cứ lúc nào trong Cài đặt."
+  "You can change this anytime in Settings.": "Bạn có thể thay đổi tùy chọn này bất cứ lúc nào trong Cài đặt.",
+  "Then by...": "Sau đó theo..."
 }

apps/readest-app/public/locales/zh-CN/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "不包含书籍内容或阅读数据",
   "Share anonymous data": "分享匿名数据",
   "Not now": "暂不",
-  "You can change this anytime in Settings.": "您可以随时在“设置”中更改此选项。"
+  "You can change this anytime in Settings.": "您可以随时在“设置”中更改此选项。",
+  "Then by...": "其次按..."
 }

apps/readest-app/public/locales/zh-TW/translation.json

@@ -1623,5 +1623,6 @@
   "No book content or reading data": "不包含書籍內容或閱讀資料",
   "Share anonymous data": "分享匿名資料",
   "Not now": "暫不",
-  "You can change this anytime in Settings.": "您可以隨時在「設定」中變更此項目。"
+  "You can change this anytime in Settings.": "您可以隨時在「設定」中變更此項目。",
+  "Then by...": "其次按..."
 }

apps/readest-app/src/__tests__/utils/library-utils.test.ts

@@ -7,7 +7,10 @@ import {
   getGroupSortValue,
   createBookSorter,
   ensureLibrarySortByType,
+  ensureLibrarySecondarySortByType,
   ensureLibraryGroupByType,
+  resolveEffectivePrimarySort,
+  resolveEffectiveSecondarySort,
   findGroupById,
   getGroupDisplayName,
   expandBookshelfSelection,
@@ -1114,3 +1117,179 @@ describe('buildGroupNameUpdatedAt', () => {
     expect(groups.map((g) => g.name)).toEqual(['Newest', 'Newer', 'Old']);
   });
 });
+
+describe('secondary sort - createBookSorter tiebreaker', () => {
+  it('uses secondary key as tiebreaker when primary keys are equal', () => {
+    const books = [
+      createMockBook({ hash: 'a', title: 'Zebra', author: 'Same Author' }),
+      createMockBook({ hash: 'b', title: 'Apple', author: 'Same Author' }),
+    ];
+    const sorter = createBookSorter(LibrarySortByType.Author, 'en', LibrarySortByType.Title);
+    const sorted = [...books].sort(sorter);
+    expect(sorted[0]!.title).toBe('Apple');
+    expect(sorted[1]!.title).toBe('Zebra');
+  });
+
+  it('falls back to default when primary differs (secondary not used)', () => {
+    const books = [
+      createMockBook({ hash: 'a', title: 'Zebra', author: 'B Author' }),
+      createMockBook({ hash: 'b', title: 'Apple', author: 'A Author' }),
+    ];
+    const sorter = createBookSorter(LibrarySortByType.Author, 'en', LibrarySortByType.Title);
+    const sorted = [...books].sort(sorter);
+    expect(sorted[0]!.author).toBe('A Author');
+    expect(sorted[1]!.author).toBe('B Author');
+  });
+
+  it('ignores secondary when set to none', () => {
+    const books = [
+      createMockBook({ hash: 'a', title: 'Zebra', author: 'Same' }),
+      createMockBook({ hash: 'b', title: 'Apple', author: 'Same' }),
+    ];
+    const sorter = createBookSorter(LibrarySortByType.Author, 'en', 'none');
+    const sorted = [...books].sort(sorter);
+    // Stable: both equal, original order preserved
+    expect(sorted[0]!.hash).toBe('a');
+    expect(sorted[1]!.hash).toBe('b');
+  });
+});
+
+describe('secondary sort - createWithinGroupSorter (drilled-in group)', () => {
+  it('inside an author group, sorts by series index when secondary is series', () => {
+    const books = [
+      createMockBook({
+        hash: '1',
+        title: 'Zebra Book',
+        metadata: { series: 'Series A', seriesIndex: 3 },
+      }),
+      createMockBook({
+        hash: '2',
+        title: 'Apple Book',
+        metadata: { series: 'Series A', seriesIndex: 1 },
+      }),
+      createMockBook({
+        hash: '3',
+        title: 'Mango Book',
+        metadata: { series: 'Series A', seriesIndex: 2 },
+      }),
+    ];
+    const sorter = createWithinGroupSorter(
+      LibraryGroupByType.Author,
+      LibrarySortByType.Title,
+      'en',
+      true,
+      LibrarySortByType.Series,
+    );
+    const sorted = [...books].sort(sorter);
+    expect(sorted.map((b) => b.metadata?.seriesIndex)).toEqual([1, 2, 3]);
+  });
+
+  it('falls back to primary when secondary ties (same series index)', () => {
+    const books = [
+      createMockBook({
+        hash: '1',
+        title: 'Zebra',
+        metadata: { seriesIndex: 1 },
+      }),
+      createMockBook({
+        hash: '2',
+        title: 'Apple',
+        metadata: { seriesIndex: 1 },
+      }),
+    ];
+    const sorter = createWithinGroupSorter(
+      LibraryGroupByType.Author,
+      LibrarySortByType.Title,
+      'en',
+      true,
+      LibrarySortByType.Series,
+    );
+    const sorted = [...books].sort(sorter);
+    expect(sorted[0]!.title).toBe('Apple');
+    expect(sorted[1]!.title).toBe('Zebra');
+  });
+
+  it('without secondary, keeps existing primary behavior in author group', () => {
+    const books = [
+      createMockBook({ hash: '1', title: 'Zebra' }),
+      createMockBook({ hash: '2', title: 'Apple' }),
+    ];
+    const sorter = createWithinGroupSorter(
+      LibraryGroupByType.Author,
+      LibrarySortByType.Title,
+      'en',
+      true,
+      'none',
+    );
+    const sorted = [...books].sort(sorter);
+    expect(sorted[0]!.title).toBe('Apple');
+    expect(sorted[1]!.title).toBe('Zebra');
+  });
+});
+
+describe('resolveEffectiveSecondarySort smart defaults', () => {
+  it('returns explicit secondary when not none', () => {
+    expect(resolveEffectiveSecondarySort(LibrarySortByType.Title, LibraryGroupByType.Author)).toBe(
+      LibrarySortByType.Title,
+    );
+  });
+
+  it('defaults to series when groupBy=Author and stored=none', () => {
+    expect(resolveEffectiveSecondarySort('none', LibraryGroupByType.Author)).toBe(
+      LibrarySortByType.Series,
+    );
+  });
+
+  it('stays none for groupBy=Series (series-index handles ordering)', () => {
+    expect(resolveEffectiveSecondarySort('none', LibraryGroupByType.Series)).toBe('none');
+  });
+
+  it('stays none for groupBy=None/Group', () => {
+    expect(resolveEffectiveSecondarySort('none', LibraryGroupByType.None)).toBe('none');
+    expect(resolveEffectiveSecondarySort('none', LibraryGroupByType.Group)).toBe('none');
+  });
+});
+
+describe('resolveEffectivePrimarySort smart defaults', () => {
+  it('returns stored when isAuto=false, regardless of groupBy', () => {
+    expect(
+      resolveEffectivePrimarySort(LibrarySortByType.Updated, LibraryGroupByType.Series, false),
+    ).toBe(LibrarySortByType.Updated);
+    expect(
+      resolveEffectivePrimarySort(LibrarySortByType.Title, LibraryGroupByType.Author, false),
+    ).toBe(LibrarySortByType.Title);
+  });
+
+  it('defaults to Series when isAuto=true and groupBy=Series', () => {
+    expect(
+      resolveEffectivePrimarySort(LibrarySortByType.Updated, LibraryGroupByType.Series, true),
+    ).toBe(LibrarySortByType.Series);
+  });
+
+  it('returns stored for non-series groupBy when isAuto=true', () => {
+    expect(
+      resolveEffectivePrimarySort(LibrarySortByType.Updated, LibraryGroupByType.Author, true),
+    ).toBe(LibrarySortByType.Updated);
+    expect(
+      resolveEffectivePrimarySort(LibrarySortByType.Updated, LibraryGroupByType.Group, true),
+    ).toBe(LibrarySortByType.Updated);
+    expect(
+      resolveEffectivePrimarySort(LibrarySortByType.Updated, LibraryGroupByType.None, true),
+    ).toBe(LibrarySortByType.Updated);
+  });
+});
+
+describe('ensureLibrarySecondarySortByType', () => {
+  it('accepts none', () => {
+    expect(ensureLibrarySecondarySortByType('none', 'none')).toBe('none');
+  });
+  it('accepts any valid LibrarySortByType', () => {
+    expect(ensureLibrarySecondarySortByType('series', 'none')).toBe(LibrarySortByType.Series);
+  });
+  it('falls back when invalid', () => {
+    expect(ensureLibrarySecondarySortByType('bogus', LibrarySortByType.Title)).toBe(
+      LibrarySortByType.Title,
+    );
+    expect(ensureLibrarySecondarySortByType(null, 'none')).toBe('none');
+  });
+});

apps/readest-app/src/app/library/components/Bookshelf.tsx

@@ -36,10 +36,13 @@ import {
   createWithinGroupSorter,
   ensureLibraryGroupByType,
   ensureLibrarySortByType,
+  ensureLibrarySecondarySortByType,
   expandBookshelfSelection,
   getBookSortValue,
   getGroupSortValue,
   compareSortValues,
+  resolveEffectivePrimarySort,
+  resolveEffectiveSecondarySort,
 } from '../utils/libraryUtils';
 import { eventDispatcher } from '@/utils/event';
 
@@ -155,9 +158,16 @@ const Bookshelf: React.FC<BookshelfProps> = ({
   const groupId = searchParams?.get('group') || '';
   const queryTerm = searchParams?.get('q') || null;
   const viewMode = searchParams?.get('view') || settings.libraryViewMode;
-  const sortBy = ensureLibrarySortByType(searchParams?.get('sort'), settings.librarySortBy);
+  const storedSortBy = ensureLibrarySortByType(searchParams?.get('sort'), settings.librarySortBy);
   const sortOrder = searchParams?.get('order') || (settings.librarySortAscending ? 'asc' : 'desc');
   const groupBy = ensureLibraryGroupByType(searchParams?.get('groupBy'), settings.libraryGroupBy);
+  const sortByAuto = settings.librarySortByAuto ?? true;
+  const sortBy = resolveEffectivePrimarySort(storedSortBy, groupBy, sortByAuto);
+  const sortBy2Raw = ensureLibrarySecondarySortByType(
+    searchParams?.get('sort2'),
+    settings.librarySortBy2 ?? 'none',
+  );
+  const sortBy2 = resolveEffectiveSecondarySort(sortBy2Raw, groupBy);
   const coverFit = searchParams?.get('cover') || settings.libraryCoverFit;
 
   const [loading, setLoading] = useState(false);
@@ -260,14 +270,20 @@ const Bookshelf: React.FC<BookshelfProps> = ({
     // Sort books within each group
     // For series groups, series index is always ascending; sort direction applies to fallback only
     const sortAscending = sortOrder === 'asc';
-    const withinGroupSorter = createWithinGroupSorter(groupBy, sortBy, uiLanguage, sortAscending);
+    const withinGroupSorter = createWithinGroupSorter(
+      groupBy,
+      sortBy,
+      uiLanguage,
+      sortAscending,
+      sortBy2,
+    );
     groups.forEach((group) => {
       group.books.sort(withinGroupSorter);
     });
 
     // Sort ungrouped books - use within-group sorter if we're inside a group
     // (for series, this ensures books are sorted by series index)
-    const bookSorter = createBookSorter(sortBy, uiLanguage);
+    const bookSorter = createBookSorter(sortBy, uiLanguage, sortBy2);
     if (groupId && groupBy !== LibraryGroupByType.Group && groupBy !== LibraryGroupByType.None) {
       ungroupedBooks.sort(withinGroupSorter);
       // When inside a group, books are already sorted correctly — return directly
@@ -309,7 +325,7 @@ const Bookshelf: React.FC<BookshelfProps> = ({
     });
 
     return allItems;
-  }, [sortOrder, sortBy, groupBy, groupId, uiLanguage, currentBookshelfItems]);
+  }, [sortOrder, sortBy, sortBy2, groupBy, groupId, uiLanguage, currentBookshelfItems]);
 
   useEffect(() => {
     if (isImportingBook.current) return;

apps/readest-app/src/app/library/components/ViewMenu.tsx

@@ -7,6 +7,7 @@ import {
   LibraryCoverFitType,
   LibraryViewModeType,
   LibraryGroupByType,
+  LibrarySecondarySortByType,
   LibrarySortByType,
 } from '@/types/settings';
 import { saveSysSettings } from '@/helpers/settings';
@@ -33,6 +34,22 @@ const ViewMenu: React.FC<ViewMenuProps> = ({ setIsDropdownOpen }) => {
   const groupBy = settings.libraryGroupBy;
   const sortBy = settings.librarySortBy;
   const isAscending = settings.librarySortAscending;
+  const sortByAuto = settings.librarySortByAuto ?? true;
+  // Primary smart default: when auto is on, grouping by Series implies Series as
+  // the primary sort. The stored value is left alone — that way turning auto off
+  // later restores the user's previous explicit pick.
+  const primaryEffective: LibrarySortByType =
+    sortByAuto && groupBy === LibraryGroupByType.Series ? LibrarySortByType.Series : sortBy;
+  const primaryIsImplicit = sortByAuto && primaryEffective !== sortBy;
+  const sortBy2: LibrarySecondarySortByType = settings.librarySortBy2 ?? 'none';
+  // Smart default: when grouping by Author and the user hasn't picked an explicit
+  // secondary, Series is implied. Surface this in the menu so the highlighted row
+  // matches the actual sort behavior.
+  const secondaryEffective: LibrarySecondarySortByType =
+    sortBy2 === 'none' && groupBy === LibraryGroupByType.Author
+      ? LibrarySortByType.Series
+      : sortBy2;
+  const secondaryIsImplicit = sortBy2 === 'none' && secondaryEffective !== 'none';
 
   const viewOptions = [
     { label: _('List'), value: 'list' },
@@ -61,6 +78,11 @@ const ViewMenu: React.FC<ViewMenuProps> = ({ setIsDropdownOpen }) => {
     { label: _('Date Published'), value: LibrarySortByType.Published },
   ];
 
+  const sortBy2Options: { label: string; value: LibrarySecondarySortByType }[] = [
+    { label: _('None'), value: 'none' },
+    ...sortByOptions,
+  ];
+
   const sortingOptions = [
     { label: _('Ascending'), value: true },
     { label: _('Descending'), value: false },
@@ -108,6 +130,9 @@ const ViewMenu: React.FC<ViewMenuProps> = ({ setIsDropdownOpen }) => {
 
   const handleSetSortBy = async (value: LibrarySortByType) => {
     await saveSysSettings(envConfig, 'librarySortBy', value);
+    // Any explicit primary pick locks in the choice and disables the auto
+    // smart-default so future groupBy changes don't override the user.
+    await saveSysSettings(envConfig, 'librarySortByAuto', false);
 
     const params = new URLSearchParams(searchParams?.toString());
     params.set('sort', value);
@@ -122,6 +147,18 @@ const ViewMenu: React.FC<ViewMenuProps> = ({ setIsDropdownOpen }) => {
     navigateToLibrary(router, `${params.toString()}`);
   };
 
+  const handleSetSortBy2 = async (value: LibrarySecondarySortByType) => {
+    await saveSysSettings(envConfig, 'librarySortBy2', value);
+
+    const params = new URLSearchParams(searchParams?.toString());
+    if (value === 'none') {
+      params.delete('sort2');
+    } else {
+      params.set('sort2', value);
+    }
+    navigateToLibrary(router, `${params.toString()}`);
+  };
+
   return (
     <Menu
       className='view-menu dropdown-content no-triangle z-20 mt-2 shadow-2xl'
@@ -202,16 +239,20 @@ const ViewMenu: React.FC<ViewMenuProps> = ({ setIsDropdownOpen }) => {
       <hr aria-hidden='true' className='border-base-200 my-1' />
       <MenuItem label={_('Sort by...')} detailsOpen={false} buttonClass='py-[4px]'>
         <ul className='ms-0 flex flex-col ps-0 before:hidden'>
-          {sortByOptions.map((option) => (
-            <MenuItem
-              key={option.value}
-              label={option.label}
-              buttonClass='h-8'
-              toggled={sortBy === option.value}
-              onClick={() => handleSetSortBy(option.value as LibrarySortByType)}
-              transient
-            />
-          ))}
+          {sortByOptions.map((option) => {
+            const isImplicit = primaryIsImplicit && option.value === primaryEffective;
+            const toggled = isImplicit || (!primaryIsImplicit && sortBy === option.value);
+            return (
+              <MenuItem
+                key={option.value}
+                label={isImplicit ? `${option.label} (${_('Auto')})` : option.label}
+                buttonClass='h-8'
+                toggled={toggled}
+                onClick={() => handleSetSortBy(option.value as LibrarySortByType)}
+                transient
+              />
+            );
+          })}
           <hr aria-hidden='true' className='border-base-200 my-1' />
           {sortingOptions.map((option) => (
             <MenuItem
@@ -225,6 +266,27 @@ const ViewMenu: React.FC<ViewMenuProps> = ({ setIsDropdownOpen }) => {
           ))}
         </ul>
       </MenuItem>
+
+      {/* Then by - secondary sort, collapsible */}
+      <hr aria-hidden='true' className='border-base-200 my-1' />
+      <MenuItem label={_('Then by...')} detailsOpen={false} buttonClass='py-[4px]'>
+        <ul className='ms-0 flex flex-col ps-0 before:hidden'>
+          {sortBy2Options.map((option) => {
+            const isImplicit = secondaryIsImplicit && option.value === secondaryEffective;
+            const isExplicit = sortBy2 === option.value;
+            return (
+              <MenuItem
+                key={option.value}
+                label={isImplicit ? `${option.label} (${_('Auto')})` : option.label}
+                buttonClass='h-8'
+                toggled={isExplicit || isImplicit}
+                onClick={() => handleSetSortBy2(option.value)}
+                transient
+              />
+            );
+          })}
+        </ul>
+      </MenuItem>
     </Menu>
   );
 };

apps/readest-app/src/app/library/utils/libraryUtils.ts

@@ -1,5 +1,9 @@
 import { Book, BooksGroup } from '@/types/book';
-import { LibraryGroupByType, LibrarySortByType } from '@/types/settings';
+import {
+  LibraryGroupByType,
+  LibrarySecondarySortByType,
+  LibrarySortByType,
+} from '@/types/settings';
 import { formatAuthors, formatTitle } from '@/utils/book';
 import { md5Fingerprint } from '@/utils/md5';
 
@@ -23,6 +27,65 @@ export const ensureLibrarySortByType = (
   return fallback;
 };
 
+/**
+ * Safely cast a query parameter to LibrarySecondarySortByType with fallback.
+ * Accepts any valid primary sort type plus the literal 'none'.
+ */
+export const ensureLibrarySecondarySortByType = (
+  value: string | null | undefined,
+  fallback: LibrarySecondarySortByType,
+): LibrarySecondarySortByType => {
+  if (value === 'none') return 'none';
+  if (value && VALID_SORT_TYPES.includes(value as LibrarySortByType)) {
+    return value as LibrarySortByType;
+  }
+  return fallback;
+};
+
+/**
+ * Resolve the *effective* primary sort key, applying smart defaults derived
+ * from the current `groupBy`. The stored `librarySortBy` is left unchanged;
+ * this only substitutes a sensible implicit default when the user is still on
+ * auto, so grouping by Series lands an alphabetical-by-series-name listing
+ * without any extra clicks.
+ *
+ * - !isAuto                              -> use stored as-is
+ * - groupBy=Series and isAuto            -> Series
+ * - everything else                      -> stored
+ */
+export const resolveEffectivePrimarySort = (
+  stored: LibrarySortByType,
+  groupBy: LibraryGroupByType,
+  isAuto: boolean,
+): LibrarySortByType => {
+  if (!isAuto) return stored;
+  if (groupBy === LibraryGroupByType.Series) return LibrarySortByType.Series;
+  return stored;
+};
+
+/**
+ * Resolve the *effective* secondary sort key, applying smart defaults derived
+ * from the current `groupBy`. The stored secondary stays whatever the user
+ * picked; this only substitutes 'none' with a sensible implicit default so
+ * users get useful behavior out of the box (e.g. drilling into an Author
+ * group lands a series-ordered list without any extra clicks).
+ *
+ * - explicit secondary (any non-'none')  -> use as-is
+ * - groupBy=Author and stored='none'     -> Series
+ * - everything else                      -> 'none'
+ *
+ * groupBy=Series doesn't default to anything because `createWithinGroupSorter`
+ * already orders by `seriesIndex` for series groups.
+ */
+export const resolveEffectiveSecondarySort = (
+  secondary: LibrarySecondarySortByType,
+  groupBy: LibraryGroupByType,
+): LibrarySecondarySortByType => {
+  if (secondary !== 'none') return secondary;
+  if (groupBy === LibraryGroupByType.Author) return LibrarySortByType.Series;
+  return 'none';
+};
+
 /**
  * Safely cast a query parameter to LibraryGroupByType with fallback.
  * Returns the value if valid, otherwise returns the fallback.
@@ -115,16 +178,18 @@ export const createBookFilter = (queryTerm: string | null) => (item: Book) => {
   );
 };
 
-export const createBookSorter = (sortBy: string, uiLanguage: string) => (a: Book, b: Book) => {
+const compareBookByKey = (a: Book, b: Book, sortBy: string, uiLanguage: string): number => {
   switch (sortBy) {
-    case LibrarySortByType.Title:
+    case LibrarySortByType.Title: {
       const aTitle = formatTitle(a.title);
       const bTitle = formatTitle(b.title);
       return aTitle.localeCompare(bTitle, uiLanguage || navigator.language);
-    case LibrarySortByType.Author:
+    }
+    case LibrarySortByType.Author: {
       const aAuthors = formatAuthors(a.author, a?.primaryLanguage || 'en', true);
       const bAuthors = formatAuthors(b.author, b?.primaryLanguage || 'en', true);
       return aAuthors.localeCompare(bAuthors, uiLanguage || navigator.language);
+    }
     case LibrarySortByType.Updated:
       return new Date(a.updatedAt).getTime() - new Date(b.updatedAt).getTime();
     case LibrarySortByType.Created:
@@ -133,7 +198,7 @@ export const createBookSorter = (sortBy: string, uiLanguage: string) => (a: Book
       return a.format.localeCompare(b.format, uiLanguage || navigator.language);
     case LibrarySortByType.Series:
       return (a.metadata?.seriesIndex || 0) - (b.metadata?.seriesIndex || 0);
-    case LibrarySortByType.Published:
+    case LibrarySortByType.Published: {
       const aPublished = a.metadata?.published || '0001-01-01';
       const bPublished = b.metadata?.published || '0001-01-01';
 
@@ -154,11 +219,25 @@ export const createBookSorter = (sortBy: string, uiLanguage: string) => (a: Book
       if (isNaN(bDate)) return -1;
 
       return aDate - bDate;
+    }
     default:
       return new Date(a.updatedAt).getTime() - new Date(b.updatedAt).getTime();
   }
 };
 
+/**
+ * @param secondarySortBy - Optional tiebreaker key applied when the primary
+ *   comparison returns 0. Pass `'none'` (or omit) to disable. Series-index ties
+ *   on a Series secondary fall through to the underlying primary tie.
+ */
+export const createBookSorter =
+  (sortBy: string, uiLanguage: string, secondarySortBy: LibrarySecondarySortByType = 'none') =>
+  (a: Book, b: Book): number => {
+    const primary = compareBookByKey(a, b, sortBy, uiLanguage);
+    if (primary !== 0 || secondarySortBy === 'none') return primary;
+    return compareBookByKey(a, b, secondarySortBy, uiLanguage);
+  };
+
 /**
  * Build a `groupName -> max(book.updatedAt)` map for all groups touched by
  * the given books. Each book bumps both its direct group and every ancestor
@@ -319,10 +398,14 @@ const createAuthorGroups = (books: Book[]): (Book | BooksGroup)[] => {
 /**
  * Create a sorter for books within a group.
  * For series groups: sort by seriesIndex first (always ascending), then by global sort for items without index.
- * For author groups: follow global sort setting.
- * @param sortAscending - When true (default), sort direction is ascending. Series index is always
- *   ascending regardless of this flag; the flag only affects the fallback sort for books without
- *   series index and non-series groupings.
+ * For other groupings: when a secondary key is supplied, sort by secondary key first (always ascending),
+ *   with the primary global sort as tiebreaker. Without secondary, follow global sort setting.
+ * @param sortAscending - When true (default), sort direction is ascending. Series index and the
+ *   secondary key are always ascending regardless of this flag; the flag affects the fallback /
+ *   primary tiebreaker only.
+ * @param secondarySortBy - When non-'none', acts as the *primary* within-group ordering for
+ *   non-series groupings (matches the user's mental model: "group by author, then sort by series"
+ *   should land series order inside each author).
  */
 export const createWithinGroupSorter =
   (
@@ -330,6 +413,7 @@ export const createWithinGroupSorter =
     sortBy: LibrarySortByType,
     uiLanguage: string,
     sortAscending: boolean = true,
+    secondarySortBy: LibrarySecondarySortByType = 'none',
   ) =>
   (a: Book, b: Book): number => {
     const sortDirection = sortAscending ? 1 : -1;
@@ -351,7 +435,14 @@ export const createWithinGroupSorter =
       return createBookSorter(sortBy, uiLanguage)(a, b) * sortDirection;
     }
 
-    // For author and other groupings, use global sort with direction
+    // For author and other non-series groupings: when a secondary key is provided,
+    // use it as the within-group primary order with the global key as tiebreaker.
+    if (secondarySortBy !== 'none') {
+      const bySecondary = compareBookByKey(a, b, secondarySortBy, uiLanguage);
+      if (bySecondary !== 0) return bySecondary;
+      return createBookSorter(sortBy, uiLanguage)(a, b) * sortDirection;
+    }
+
     return createBookSorter(sortBy, uiLanguage)(a, b) * sortDirection;
   };
 

apps/readest-app/src/services/constants.ts

@@ -121,6 +121,8 @@ export const DEFAULT_SYSTEM_SETTINGS: Partial<SystemSettings> = {
   libraryViewMode: 'grid',
   librarySortBy: LibrarySortByType.Updated,
   librarySortAscending: false,
+  librarySortByAuto: true,
+  librarySortBy2: 'none',
   libraryGroupBy: LibraryGroupByType.Group,
   libraryCoverFit: 'crop',
   libraryAutoColumns: true,

apps/readest-app/src/types/settings.ts

@@ -22,6 +22,15 @@ export const LibrarySortByType = {
 
 export type LibrarySortByType = (typeof LibrarySortByType)[keyof typeof LibrarySortByType];
 
+/**
+ * Secondary sort key. Same options as the primary sort key plus `'none'` which
+ * disables the secondary sort. When set to `'none'` and a smart default applies
+ * (e.g. groupBy=Author -> series), the resolver in `libraryUtils` substitutes
+ * the implicit default at sort time without persisting it. See
+ * `resolveEffectiveSecondarySort`.
+ */
+export type LibrarySecondarySortByType = LibrarySortByType | 'none';
+
 export type LibraryCoverFitType = 'crop' | 'fit';
 
 export const LibraryGroupByType = {
@@ -288,6 +297,15 @@ export interface SystemSettings {
   libraryViewMode: LibraryViewModeType;
   librarySortBy: LibrarySortByType;
   librarySortAscending: boolean;
+  /**
+   * Whether the primary sort uses a smart default derived from `libraryGroupBy`.
+   * When `true` and grouping by Series, the effective primary sort becomes
+   * Series at sort time (the stored `librarySortBy` is left unchanged so users
+   * who later turn auto off keep their previous explicit pick). Flipped to
+   * `false` the moment the user picks any primary sort in the menu.
+   */
+  librarySortByAuto: boolean;
+  librarySortBy2: LibrarySecondarySortByType;
   libraryGroupBy: LibraryGroupByType;
   libraryCoverFit: LibraryCoverFitType;
   libraryAutoColumns: boolean;