There are, I believe, some books that are so well written that making a movie that does them justice is just not possible...reason being, books don't have real people playing out their parts or actual visual scenes and stuff. The author writes and the reader imagines.....I'm not saying writing books is easier than making movies, but IMO writing bestsellers is easier than making awesome movies.
Then again some books are easier to make into a movie......I mean, I recently saw 'Up in the Air'....liked it a lot. And like it's been mentioned, The Godfather, that's one movie that is actually better than the book. Good book, but awesome movie.