Likely called "Compendium". I'm torn on whether this should include the UI or should be a slightly more abstract setup. The abstract version would still have a lot of the logic that would back a UI, but the library user would be free to configure how the UI actually appears.

Imo convenience is better. The last guide book was a flawless plug n play ana made things... ludicrously convenient.
If a mod wants something different, they can do it themselves, or even just so their own UI but using the liblib logic. They shouldn't be coupled at all.

So basically both? An abstract system with a default GUI implementation?

Yes, the json system should be easily portable from. 1.12 since it's very decoupled and relies on Lang files.