The Exorcist - William Peter Blatty
Thrilling and splendid.
You will finish this novel in one sitting, I did. It is so captivating that you just can't put it down. We all saw the movie and it was great yet scary but this novel it above and beyond, it really plays with your mind. It makes you think that maybe just maybe she does just have a mental illness. It shows how easy a possession can be misdiagnosed as a mental illness and visa versa.
This is really one of those novels that stays with you.