So there is a mod called patchouli,https://www.curseforge.com/minecraft/mc-mods/patchouli
which can be used to make documentary books by simple JSON code with lots of features like pictures and localisation and bookmarks.
It looks fairly simple so I think this should be added in to mcreator so we can have a actual book mod element.
Issue comments
As Klemen said, you can already add Patchouli support yourself. It isn't hard and there is a guide.
I also tried once, but there was too much work to be done to create a guidebook for RaolCraft and Patchouli was kinda bugged and caused random crashes (it probably was because it was just released in beta, it probably does no longer crash now)
(this screenshot is very old, it was made while 2.0 part 4 was being developed in November 2018... XD)
We don't plan to support APIs yet, but will do later down the road.
If the format is pure JSON, you can simply list the mod as a dependency and manually add JSON files required by the mod.
MCreator 2019.5 will support adding new JSON files.
PS: This is not a bug report as you marked.