An author sells the movie rights of the book. Unless an author has "final script approval" built into their contract, it's out of their hands. One of my favorite authors is Clive Cussler. He sold "Raise the Titanic". He didn't have final script approval and he hated the movie. So dissappointed in it that he swore he would never allow a book of his to be made into a movie. He has since relented and sold, obviously, "Sahara", which starred Matthew McConaughey. He did, however, have final script approval for it and he also had a hand in picking the cast. Even so, I believe he ended up suing the producers over something connected with it. He also sold another book, but who knows if it will ever be made into the movie.
Example 2: My sister's favorite movie is "Jaws". Last summer we decided to read the book because we never had even though we've seen the movie a million times. The book is very short and barely centered around the shark. The movie is completely different from the book - and the movie is much better I might add. You just never know what you're going to get. In most cases, however, I find the book is better.