I note that Guidebook#buildBook() is currently called from Guide-API's preInit(). This doesn't quite work with the new Forge registry system; if I have a registry class in my mod using @ObjectHolder to inject my blocks and items, and using registry events to register those blocks/items, that doesn't happen until after preInit(). So any block or item fields in my registry class are null at the point that Guidebook#buildBook() is called, which is a problem for using those blocks/items as category icons.

I suppose buildBook() needs to be called from init() rather than preInit()?

This affects way more than just entry/category items, it's also a problem for any recipe involving a modded ingredient/product. This needs to be fixed ASAP.

...I thought I fixed this. Maybe I just didn't push it?

Aight. That commit should fix this.