Actually this answer seems more useful than presumed initially. I only tried this method on one book, and that book's conversion looked bad. I have tried multiple other books now, and most look reasonably well after conversion. Still, it might be possible to make the conversion look even better using mutool its epub/css config options.
Although maybe not a very useful answer here, I still like to inform about the following possibility.
Emacs comes with doc-view-mode already. If you have the
mutool command available on your system then Emacs doc-view-mode will use it to convert pdf's to png.
Now mutool is also able to convert epub to png. So by making doc-view to recognize an epub as a pdf, i.e. add
("epub" pdf) to the pdf options in the
doc-view-set-doc-type function, you can read an epub by opening your epub file and then switch to doc-view-mode. The conversion takes some time, and it takes a while (~30sec) before the png image will appear.
Now I tried this succesfully myself, but the converted document did not look great. However, the
mutool draw command offers some configuration options for epub conversion. Unfortunately I do not understand these options well yet, as mutool tells me the following when using a very simple css stylesheet.
error: css syntax error: expected keyword in property (content/stylesheet.css:3)
Maybe someone who understands mupdf or css or epub could comment on this answer for how to get a nice epub conversion with mutool. Then doc-view-mode might be another nice option for reading epubs (and a feature request could be opened for official epub support in doc-view-mode).