Sorry this one is a day late everybody but I had a blinder of a headache last night and a computer screen was the last thing I wanted to see. Not that you've really missed out on anything because I honestly can't think of a book that really disturbed me. Unless by disturbed you mean scared, in which case 90% of Stephen King's novels have done that, as well as a myriad of other horror novels.
Actually now that I've said that I was fairly disturbed by It by Mr King. I saw the movie (TV special?) of it and was petrified of Tim Curry's Pennywise and for who knows what reason decided to hire out the book from the library. I must have been about 12 at this stage and I remember reading half of the first chapter before visions of Tim Curry's creepy smile kept floating across my vision and I threw the book across the room. The next day I returned it and I've never been brave enough to pick it back up.