diff --git a/.skills/SKILL.md b/.skills/SKILL.md
index 1e687b3e..3d34eaca 100644
--- a/.skills/SKILL.md
+++ b/.skills/SKILL.md
@@ -847,8 +847,8 @@ rm -rf /path/to/sd/.crosspoint/epub_<hash>/sections/
 **Source**: `lib/Epub/Epub/Section.cpp`, `lib/Epub/Epub/BookMetadataCache.cpp`
 
 **Current Versions** (as of docs/file-formats.md):
-- `book.bin`: **Version 5** (metadata structure)
-- `section.bin`: **Version 20** (layout structure, includes paragraph LUT)
+- `book.bin`: **Version 7** (header A includes cached `tocReliable` byte)
+- `section.bin`: **Version 23** (layout structure, includes paragraph LUT)
 
 **Version Increment Rules**:
 1. **ALWAYS increment version** BEFORE changing binary structure
diff --git a/docs/epub-toc-navigation.md b/docs/epub-toc-navigation.md
index 2e623c65..6f40af82 100644
--- a/docs/epub-toc-navigation.md
+++ b/docs/epub-toc-navigation.md
@@ -22,6 +22,14 @@ Spine items before the first TOC entry (cover pages) and after the last (appendi
 - `getTocItem(i)` returns the TOC entry (title, spineIndex, anchor) for TOC index `i` -- also a file seek per call, not cached in memory. Code that queries TOC metadata in a loop should cache the results locally first.
 - `getSpineIndexForTocIndex(i)` does the reverse lookup (TOC index to spine index).
 
+### Cached TOC reliability flag
+
+`hasReliableToc()` answers whether the TOC has enough spine coverage (>=25% of spines referenced) to drive chapter UX, with short-circuits for `tocCount <= 0` and the "large book with one TOC entry" pathology.
+
+The result is computed once during `buildBookBin` (folded into the existing `spineIndex->tocIndex` scan, so no extra disk pass) and persisted as a single byte in book.bin's header A. `Epub::hasReliableToc()` reads `BookMetadataCache::isTocReliable()` and caches the bool in `tocReliabilityState`.
+
+This matters because the check used to recompute the answer on demand by calling `getTocEntry(i)` for every TOC entry, which does two SD-card seeks per call. On a 2858-entry web-novel TOC that was ~5700 seeks (~7 seconds) added to first-page latency. `BOOK_CACHE_VERSION` was bumped to 7 for this layout change; older caches are rebuilt on next open.
+
 ## Section cache file format
 
 The section cache (`.bin`) stores pre-rendered page data for a spine item. The file layout:
diff --git a/docs/file-formats.md b/docs/file-formats.md
index 61c4c8bd..669ae078 100644
--- a/docs/file-formats.md
+++ b/docs/file-formats.md
@@ -2,7 +2,7 @@
 
 ## `book.bin`
 
-### Version 3
+### Version 7
 
 ImHex Pattern:
 
@@ -12,7 +12,7 @@ import std.string;
 import std.core;
 
 // === Configuration ===
-#define EXPECTED_VERSION 3
+#define EXPECTED_VERSION 7
 #define MAX_STRING_LENGTH 65535
 
 // === String Structure ===
@@ -34,8 +34,12 @@ fn format_string(String s) {
 struct Metadata {
     String title [[comment("Book title")]];
     String author [[comment("Book author")]];
+    String language [[comment("BCP47 language tag")]];
     String coverItemHref [[comment("Path to cover image")]];
     String textReferenceHref [[comment("Path to guided first text reference")]];
+    String series [[comment("Series name")]];
+    String seriesIndex [[comment("Series index/position")]];
+    String description [[comment("Book description / blurb")]];
 } [[comment("Book metadata information")]];
 
 // === Spine Entry Structure ===
@@ -70,7 +74,8 @@ struct BookBin {
     u32 lutOffset [[comment("Offset to lookup tables"), color("6BCB77")]];
     u16 spineCount [[comment("Number of spine entries"), color("4D96FF")]];
     u16 tocCount [[comment("Number of TOC entries"), color("FF6B9D")]];
-    
+    u8 tocReliable [[comment("1 if TOC has >=25% spine coverage, 0 otherwise"), color("F4A261")]];
+
     // Metadata section
     Metadata metadata [[comment("Book metadata")]];