This is one of perhaps half a dozen books that actually scared the crap out of me.
This tale of the demonic possession of a young boy in 1970s upper crust New England is so horrifying because it's written in such a matter-of-fact manner. There's no projectile vomiting of pea soup or masturbating with crucifixes. Instead, it's a subtly disturbing story of mental illness and honest-to-gosh demons, who select their victims very carefully and "seduce" them into becoming vessels for absolute evil. Several events depicted in the book are based on eyewitness accounts of actual exorcisms.
(and in case anyone is curious, the other books which succeeded in scaring the metaphorical excrement from my lower intestines were: House Of Leaves, World War Z, The Great God Pan and several Lovecraft tales, particularly The Shunned House.)