Many people may not know the name, but I am pretty sure they would know the name of the movie (1973) and the book (1971) The Exorcist. Which he produced the movie as well.
William Peter Blatty created a cult favorite of a little girl in the 40s who was possessed, and was taken under the care of priests who were determined to save her.
The movie was perfect. I remember when I first saw it when I was a little girl, I was intrigued. In my late teens, I was working in the local movie theater and the movie had a re-release, and I would be the only one sitting by myself while I see others sinking low in their seats and covering their faces, and I just sat there with a grin on my face.
I, of course, loved the book as well. But seeing it on the screen made it even more terrifying. Blatty wrote other books as well, as sequels to the Exorcist, Legion, and The Redemption. He has come out with other books up to 2015, and later died in 2017.