Book to Movie

I was thinking about this topic for a while now, and I was thinking that maybe I could work off of the book to movie idea. I though that maybe I could look into how the message or point of a book is changed when it is turned into a movie. How the underlying principles that made a book so interesting are often taken away when it is turned into a film. The only problem with this is that this is more of a philisophical question, based more on opinion than anything else, and that could make it very difficult to do any research.