One of the books I've read in French was sci-fi, sort of, and not a bad read; I see now on the French Wikipedia that its author is considered a forerunner of "French sci-fi", and there is a ridiculously long analysis of the book on its Wikipedia page (full of spoilers, obviously). It's called "La Nuit des Temps" by René Barjavel, but you could try other books by him too, I suppose.

If you're into historical fiction, there's a classic series by Maurice Druon about the transition period between the Capetien and Valois houses of the French monarchy, in the first half of the 14th century, called "Les Rois Maudits". Seven books, but they're only about 250-300 pages each, if that, so it's manageable. I've read the first four or five, still have to read the final few as my library doesn't have those.

I'm afraid that's about all I can tell you as far as semi-recent (Druon's books were written between 1955 and 1977) French fiction books that I've actually read go. Oh, one more now that I think about it - Amin Maalouf, he's obviously of Arabic origin but writes in French. His "Echelles du Levant" is not bad, haven't read his other books yet.