Of course there is an emacs mode for X!
I’ve wanted to be able integrate epub and org for a long time. Emacs now has some very good integration with pdfs, bibtext and org. I’m starting to use Interleave (https://github.com/rudolfochrist/interleave), an emacs minor mode to interleave notes and text books. But but I’m finding that almost all new books are coming out in epub. So it’s good to now work with epub inside emacs as well. Wasamasa also wrote eyebrowse (https://github.com/wasamasa/eyebrowse) a emacs windows desktop manager which I couldn’t live without.
EDIT: I just installed it. It can’t open a lot of epubs that I have locally, but when it does work it’s pretty good. When I have time (ha!) I will try to track down what is giving it problems.