How to Find the Guide Book for a Specific Mod

Check the creative tab first

Guide-API puts every registered guide book into its own creative tab. Open the creative inventory and scroll the tab list until you find the Guide-API tab; the books inside are sorted by the mods that registered them. The item's name and tooltip tell you which mod it belongs to.

If you are playing Survival

Books are not always craftable. Many mods hand you the book on first join, spawn it in a starter chest, or tie it to a specific block or entity. If you cannot find it, check the mod's own advancement or quest tab, look through the recipes the mod adds in JEI/REI, and search your inventory and nearby chests by name. Some mods also let you craft the book from a common item plus something the mod adds.

If the book seems missing

A guide book only exists if the mod that owns it is installed and has registered it. If you installed a mod that you expect to have a book but the Guide-API tab is empty, the mod may not use Guide-API at all, or it may register its book only under certain conditions. Check the mod's config file for an option that enables or disables the book, and make sure you are not running a version of the mod that dropped Guide-API support.