The number one scariest book I've ever read is The Witching Hour by Anne Rice. It's the first book in her Mayfair Witches series and it scared me to death. I don't remember when I read it, I just know that I was on a plane for a large portion of it. Now granted, an airplane is not anything like the setting of this book. But the descriptions were so fantastic that it scared me witless, anyway.
Next would have to be Helter Skelter by Vincent Bugliosi - Unless you've been living under a rock, you are familiar with this book by the prosecuting attorney at Charles Manson's trial. The picture in the middle of Manson's face with the swastika carved in his forehead. That is the stuff of nightmares.
Stephen King may turn up a lot on this list. Right now I can't decide between Cujo or Pet Sematary for super scary. Both of them are about animals and that is just the worst.
I think I probably need to mention The Passage by Justin Cronin here. That book was filled with nasty things that you were just sure were going to jump out of the night and eat everyone.
The Haunting of Hill House by Shirley Jackson is probably a good one to put on the list. I found myself pretty freaked out a time or two reading that book.
The Picture of Dorian Gray by Oscar Wilde gets pretty scary there toward the end, I gotta admit.
Here's another non-fiction book: The Devil In the White City by Erik Larson - I created a shelf on my Goodreads page called "couldn't even finish" for this book. The first chapter was so creepy and violent I hid it under my couch for months.
The Dante Club by Matthew Pearl was similar. The first chapter had a very detailed and graphic description of maggots on a dead body that I had to close it tight and put the book in my freezer. 'Cause that helps.
Pretty much anything having to do with Jack the Ripper is horrifying.
Let's be honest, I don't really read that many horror books. I'm kind of impressed by the number I already have here, so I'm not going to push myself to find more.
What's on your list of scary books?