What is the shared series codex, and how is it different from a book's own Codex?

Every book has its own Codex (its story bible). A series adds a second, shared one: characters, world entries and knowledge that apply to EVERY volume of the series. Open any book of the series, click "Codex", and use the "This book / Series" switch at the top — or open the series page from the Library and click "Open the codex". Put saga-wide material (main characters, the magic system, the world map) in the series codex, and volume-specific material in each book's own. If both define the same character key, the book's own card wins for that volume. Shared characters' {{f:key}} name variables resolve in every volume automatically.