Heyo, I'm that guy who asked for help on creating a custom book texture, you asked me to post this issue so you didn't forget.
There's no step-by-step guide on how to make custom textures for your book. For people new to this like me it can be very confusing trying to figure out what directories to include in your json files. The only reason I was able to do this is by looking at the example books section, which isn't very intuitive. A guide would be nice.