You're welcome for the feedback. I saw the epub import and thought it was novel, but as you said, I don't know of anywhere to buy modern fiction in epub. If you know of legal sellers and linked to them from the site, that would probably be enough for me as a customer.

That said, I'd gladly pay you/the site to handle that for me (by paying more than the book's retail price). Hopefully the translation would also be better than anything I imported.

(Two sibling replies linked to sites that sell technical non-fiction. That is a very hard way to learn :-) )

I've bought a few epubs (English only) from https://www.humblebundle.com/books and https://www.fanatical.com/en/bundle/books . I'm still skeptical of this proposition though: to learn a language by listening to a TTS? Better to go talk to people. The language starts to solidify after the sounds and non-verbal cues start matching up.

Do you require the epub to be drm stripped first?

The app doesn't do anything to remove DRM, so it'll only work with DRM-free files.