Haunted, by Chuck Palahniuk. I was reading a ton of transgressive fiction around the same time I read American Psycho. A Clockwork Orange is another good one, and someone already mentioned Lolita.

My Sweet Audrina by V.C. Andrews is worth considering, as it's standalone (and one of the few books she actually wrote before death) and I remember it being bleaker than Flowers In The Attic.