Currently, books can be in any one creative tab, specified as a string in book.json. My suggestion is that the option of specifying multiple creative tabs, as comma separated names in the string or as a json list, where the book would be added to all listed tabs (in addition to the search tab, as currently).

This would allow books being used for multiple mods (such as with addons) to appear in the creative tab for all mods using it, and it would give developers using Patchouli to put the book in a vanilla creative tab (such as Misc) in addition to their mod tab.