you might want to consider Gaston Leroux' Fantôme de l'Opéra (yes, the book on which the musical and everything else is based), and his "Mystère de la Chambre Jaune" if you're into detective stories (it's considered one of the first detective stories involving a seemingly impossible crime within a hermetically closed room). And if you like those, you could move on to other Leroux' books, or to the books about Arsène Lupin, the "gentlemen-cambrioleur". Though I've only ever read those in comic book form, I must admit.

Though while we're on the topic of comic books, that's perhaps not such a bad idea. You could try and get some of the original French versions of Astérix? I don't think you'd be very familiar with other Franco-Belgian comic book series, but I can recommend Blake & Mortimer (the early ones, don't get any of the recent crap) and, set in Ancient Rome so perhaps interesting for you, Alix (again, the early ones, and not even all of those, but Les Légions Perdues for instance is a classic). Not sure how easy any of those will be to get hold of, but otoh, while you're ordering from Amazon.fr anyway...