Les Miserables....You have to be a good reader and be able to stick it through, but so far it's awesome. I've only read half as of now and it continues to get better and better. (The unabridged version is 1200 pages)
Great insight into life and beautiful prose. If this doesn't stimulate some in depth thought, then you're doomed to a life of nothingness.
If you want a short book, then I suggest A Clockwork Orange. It's relatively short and provides some great material to think about. The language itself makes you think.