The texture for Patchouli books in 1.16.3 is broken. I've also tried the alternate book_texture in the book.json file. This also affects all mods that use Patchouli.

Minecraft: 1.16.3
Forge: 34.1.19
Patchouli: 1.16.2-44

[m[33m[12:02:24] [Render thread/WARN] [patchouli/]: You're trying to deserialize an object of type interface net.minecraft.util.text.ITextComponent from one of type class net.minecraft.util.text.TranslationTextComponent. This is likely not what you want!

I have the same issue. Is there a way to check if this is the mod looking in the wrong place or an issue with the texture?

Just tried Patchouli-1.16.2-45 and it is still not rendering

Fixed in 1.16.2-46