I heard that when a book is developed into a movie, A LOT can change. I think they call the movie thing a "screen play"...or something like that. Whoever writes the movie, dialogue, plot, etc. is the screen writer. He/she is given all the credit for the movie.
It can just be a "shadow cast on the walk" of the original book.
The original book is just the outline by which the screen play is written. Some books never can be translated into a screenplay, and when they do, it is considered quite a feat. I think the original authors have to 'approve' or 'write off on' the screenplay. Some will, some won't; some do, some don't.