The most horrifying, disturbing novel for me is Off Season by Jack Ketchum. Published by Ballantine Books, it became a smash best-seller, but because of the enormous controversy and outcry, the company pulled it after its initial printings sold out. After that, it became a collector's obsession. Eventually, another company reissued it in its original "writer's cut" with even more explicit scenes cut by Ballantine and a much darker conclusion. If you manage to find a copy, don't read it alone at night (and day doesn't help much either) - I'm being "dead" serious.
Gerard