By which I mean scary.
I have only ever read one book that I have had a physical gut wrenching reaction to - A manual written by evangelical Christians for exorcising the mentally ill. Even that didn't scare me, just made me ill.
I am reading quite a bit of horror at the moment, and while some of it is truly good writing, it's not scaring me.
I want your recommendations for horror that will scare me witless. Give me the best book that you have read that's made you want to walk to the bedroom with the light on.