The most disturbing book i've read was by George Bataille - Story of the Eye. While he's explaining his philosophy, he does this in such a disturbing story about explicit content, which i will not describe here. I was reading this book on my way to school in the subway and just couldn't stop blushing.
So, not a good book to read when you're young and undamaged.
But besides that i also have to go with Tokyo from Mo Hayder. Everything she writes is so scary!