You evidently are a pathfinder Bob. Not much out there. 
Here is an old AI Forum post regarding epub
https://groups.google.com/g/mitappinventortest/c/ID8TXj_o2aY written in 2018 says not possible but provides some links that indicate someone could possibly write an App Inventor extension to do the heavy lifting.
A probable way to make a pdf book: How do I create a pdf book app?
Someone else might have other ideas. 