I saw it and also read the book. Please read the book.  Blatty is an
author-turned-director and writes a mean story. He did an accurate adoptation
(naturally) but I think the book had better imagery and dialogue. I don't
know if they changed the book title when the movie was made, but when I read
it in the 70's, the title was "Twinkle, Twinkle Killer Kane."