<link> and <image> tags need to be allowed to be nested inside of <p> blocks.

also could use additional tags for <b>old, <i>talics, and <u>nderline, and they should also be allowed to be nested inside of the <p> blocks.

I have released a brand-new (beta) version of Guidebook, which MAY break existing books, but among other things, it allows mixed nesting of text, spans (for formatting), stacks, and images, within p blocks.
You can find the beta release here: https://minecraft.curseforge.com/projects/guidebook/files/2518170
I do plan more improvements before I consider my v2.0 goals completed.

This is planned, but I have limited time right now. Along with <span> tags.

No, hurry, I'm just reported what I'm finding that you didn't mention was planned.

Btw, this is by far the easiest way I've seen to add a book.