I know that I mentioned this in my review the other day (here) but The Omen by David Seltzer is the scariest book I have read in a long time.
Just to recap, the story is about American diplomat Robert Thorn, who exchanges his dead child for a new-born orphan. No one, not even his wife, knows of the swap. As the boy, Damien, grows older, strange events start happening. First, his Nanny dies in strange circumstances, and then a suspicious new Nanny and ferocious black dog appear to take care of him. Kathy, Robert’s wife has a serious accident and a peculiar old priest stalks Thorn to tell him that his son is the spawn of the devil. As the deaths begin to increase, Thorn must race across Rome, Jerusalem and London to try and unravel the truth.
This book had me looking over my shoulder when I was reading it (even from the safety of my own bed!) Every time you think you know what is going to happen, Seltzer thoroughly surprises you. I find horror stories that involve children are always the scariest, and when you are dealing with the child of the devil masked behind an innocent face, it suddenly becomes much more terrifying!