About difficult books, I recommand you "Le Discours de la Méthode" of René Descartes : not only difficult to read, but also very annoying and I don't agree at all with him !
I've also tried 'Also Sprach Zarathustra' of Nietzsche : Oh God! Didn't understand a single word! Maybe it's due to the translation, but I'm not sure...
Another one I couldn't read, though considered as the Best French Writer of All The Times
Victor Hugo's Notre Dame de Paris... I've read about twenty pages and gave up when I realize he was still describing the cathedral. I don't like Hugo, I think he's pretentious and don't write so well, and what a bad poet!!!!
Another french writter I can't read is Marcel Proust, with his sentences of 10 lines ! When you reach to the end of a sentence you can't remember what he was talking about at the beginning
Finally, two things I found difficult, but different reasons : Rabelais' Gargantua and Pantagruel : it's old french and quite ununderstandable ; and Dante Alighieri's Divine Comedy : I've read it in french, in a quite bad translation, but couldn't read it in italian, simply because it's not
italian and I couldn't understand half a word !
Oh yes, another one who is quite difficult to read is Shakespeare, but I keep on reading him in bilingual editions, because I think it's not quite good in french...