My vote goes to "The Amityville Horror," by Jay Anson. Sure, I've read just about every single scary novel that Stephen King put out there, and yeah, they were scary as hell. But I always knew reading a King novel that all the things contained within them were merely figments of King's brilliant imagination.
Anson's book, however, is allegedly based on fact. It was during my pre-teenage life in the early 1980s when I decided to read "The Amityville Horror," which was based on the alleged true-life experience of the Lutz family in a haunted house in Long Island, New York. The end of every chapter featured a horrifying cliffhanger that made it difficult to sleep at night. The story took place over a 28-day period from December 1975 through January 1976 - when the Lutz family abandoned the house after enduring a month's worth of ghostly encounters. Forget the movies that attempted to map out the terror experienced by the Lutz family. Read the book instead. It will scare the hell out of you!