La collection Libertinage is a collection from La Bibliothèque électronique du Québec composed by books that are "susceptible to offend the readers' modesty", what I prefer to call naughty books. There are PDF versions of nine book, including Sade's La Philosophie dans le boudoir and Apollinaire's Les Exploits d'un jeune don Juan.

But that's not the only place to find those kind of books. The Project Gutenberg has good examples too: Aline et Valcour tome 1 and tome II by Marquis de Sade, La Pantoufle de Sapho and Venus in Furs by Leopold Ritter von Sacher-Masoch, Kamasutraby Vatsyayana, The Devil's Dictionary by Ambrose Bierce (funnier than naughty) and many others, just make a search. One last suggestion, Lady Chatterley's Lover is available at Project Gutenberg of Australia.