Code review pass (automated, thorough): no bugs found. All 51 tests + format/lint/build verified on feat/20-sorting @ 08b3aa7.

Verified in depth: sortBooks is a stable, non-mutating sort with an index tiebreaker; null publish years consistently sink to the end regardless of direction; title sorts negate localeCompare; persistence merges into existing t19.preferences and preserves unrelated keys; isSortOption guard covered by tests on both layers; the snapshot's cover→placeholder switch is expected (title-asc puts the coverless fixture first).

Non-blocking notes:

1. UX consistency (low): changing sort keeps the position rather than the book — while subject change (in feat: wire subject filter into the viewer with index reset #27) resets the index. Consider making both preferences consistent (reset on sort change too) during the feat: Wire preferences together and verify reload persistence #22 integration.
2. Deliberate? FavoritesView receives the unsorted books while navigation/jump list use sortedBooks — favorites always shows API order. Fine if intentional (PR description scopes sorting to navigation), just confirm it's conscious.
3. Maintainability: isSortOption duplicates the SortOption union — deriving it from SORT_OPTIONS (exported from utils/sortBooks) would give one source of truth when options change.
4. DEFAULT_SORT is exported but only used internally in usePreferences.ts — harmless.

Ready to merge from my perspective.