Sometimes the screenwriter does the job that the book editor SHOULD have done – eliminating unnecessary characters, and inane subplots – producing a stronger vision than the original source material. Jaws, The Godfather, Fight Club, and Little Big Man are some movies I can think of that were better than the books.
It’s been my experience that attempting to give a cat a belly rub will result in said cat closing over your hand like a Venus Flytrap. In other words, you’ll pull away a bloody stump.
It was written by Thomas Berger in 1964. It basically provided the framework for the film and not much else.