Twinkle Twinkle Killer Kane
As somebody mentioned on another thread, the movie was based on Blatty's novel "Twinkle Twinkle Killer Kane" (and this was supposed to be the original movie title). A few years after the movie came out, Blatty released a completely rewritten version of "Twinkle..." with the title "Ninth Configuration."
The movie and the rewrite were superior to "Twinkle." Most of "Twinkle" was unsuccessful slapstick comedy with a subplot involving a Bela Lugosi-type character. The philosophical/theological components there (including Kane's discussions and final communications with Cutshaw) were quite weak compared to the later version, and Kane's suicide was anti-climactic in the novel (due in part to the non-existent connection with Dr. Fell, who was a minor character and not Kane's brother in "Twinkle").
I found "Twinkle" to be a rare case where the movie is superior to the book. Apparently Blatty agreed, so much so that he felt it necessary to rewrite the "Twinkle" completely based on the movie.