Several have mentioned The Girl Next Door by Jack Ketchum. I haven't read it, but I would have to say another one he wrote called Off Season is very disturbing, brutal even. I guess I should check out The Girl Next Door now.
He's more known for his comics work, but in the early 90's, John Byrne wrote a horror novel called The Whipping Boy, that was pretty disturbing. It's not in print anymore, but if you spot it at a used bookstore, grab it!