So, I'm not sure if this could be an issue for me, or the mod, but in-game, the books don't have a model, as seen below:

Currently using Minecraft 1.12.2, Forge 14.23.1.2604, and Guidebooks 2.0.0

There is an error loading the assets it needs, here: https://pastebin.com/8FPZyXYN

Yeah, it's a well know Issue by Gigaherz; (Quote from Github)

gigaherz
Posted Jul 7, 2017
Hmm you are right, the 1.12 version seems to be unable to load textures from the config folder. I'll try to figure out why.

We're waiting for him to figure out a workaround or fix, so i can update the config and book.
Maybe the pastebin helps him figure it out.

It is a separate issue. Seems like newer versions of forge cause the mod to be unable to load the model itself (instead of just the cover). I will get to it soon.

v2.0.1 is in review on curseforge and will be available shortly. Turns out I had already fixed the bug locally, I just forgot to release it.