I've spent the afternoon writing an EPUB format writer for @novelwriter. It now has all the meta data XML files and can dump raw text of the novel without text formatting, so that's very good progress.
Next is of course to make it also format the text itself for the pages.
It's an interesting format to work with, and like ODT and DOCX is rather messy. Like DOCX, I get the feeling it's been modified a lot too.