I know you said you're not looking for public domain, but:
1) they're not all boring
2) some public domain stuff is recent (and just not copyrighted)
3) other people might be interested
So here I share with you what I think is the best ressource for ebooks in French: http://www.ebooksgratuits.com/
You can sort them by authors (including some translation of English authors, actually), by type of text (short story, novel, poetry, play...) and by genre (historical, fantasy/sf...)
If you go to "news" there will be a link to download a zip file with all the files they had available at the time, in .lrf format. (or in pdf, or for mobipocket).
Although that probably won't work for you, you probably would prefer looking around and just downloading a few here and there.
They seem to be a small team but they add new content, I check back from time to time.
The only thing is that some of the books might not be public domain depending on where you are. For such books that aren't public domain everywhere, they have a warning in the summary.
Anyway, hope you'll find something you like.