The Exorcist by William Peter Blatty
Most folks know The Exorcist for the incredibly terrifying film, but the book is right up there with it in terms of horror awesomeness. The title says it all, as a priest is tasked with performing an exorcism on a young girl who is possessed by a demon. An absolute classic, and for good reason!
Scare Factor: 5 - Absolutely terrifying, and not for the kiddies. If you consider yourself a scaredy cat, you would be advised to steer clear!