Minecraft wiki entry for Book_and_Quill, associated text: The in-game restrictions on books are not hard limits. Third party editors or commands can make books have more than 100 pages‌[Java Edition only] (2,147,483,639 pages), more than 256 characters per page (32,767 characters per page), can change author names, and can have colored titles, among other things. These books still function properly in vanilla Minecraft, meaning the player may encounter such books on adventure maps or multiplayer servers.[2]
If the player renames a book and quill in an anvil before signing it, it retains the name from the anvil instead of taking the name from signing.‌[Java Edition only]
If the player renames a book and quill in an anvil before writing in it, they cannot write anything in the book. If something is written in it before it is renamed, then it functions normally.