There are three books which still reign in my mind as the three scariest books I have ever read, and here they are(in the order I like them):
1. A Winter Haunting, by Dan Simmons
2. Blood Games, by Richard Laymon
3. The Church of Dead Girls, by Stephen Dobyns
For those who are wondering, all three of these authors are admired by Stephen King, which will tell you the quality of their work. Anyway, the books I mentioned above have disturbed me the most!
Also, the books, "Darkness Tell Us," and "In The Dark," both by Richard Laymon, will keep you awake at night, as well as the three I mentioned above. Lock the doors, turn on all the lights, and make sure you don't have to work the next day, because they will keep you awake!