 Like "The Exorcist", it involves demonic possession.The book was the focus of a court case over its exclusion from "The New York Times" Best Seller list.Blatty based aspects of the Gemini Killer on the real life Zodiac Killer, who, in a January 1974 letter to the "San Francisco Chronicle", had praised the original "Exorcist" film as "the best satirical comedy that I have ever seen".