I occasionally use epr to read epubs in a terminal. Works pretty well.

https://github.com/wustho/epr

I use console emacs and nov.el (https://depp.brause.cc/nov.el/) for this. You can then use emacs features like keyboard navigation and dictionaries.

I’ve been using epy/epr to read novels, and I think it’s great.

https://github.com/wustho/epy

I've used bk https://github.com/aeosynth/bk

Generalizing this to be an epub reader would be fantastic. I'd use it, no doubt.