It's rare, but it does happen.
The movie is better than the book.
One movie that comes to mind right away is, The Prize Winner of Defiance, Ohio. I both read the book and watched the movie. I liked the movie better.
What made it better than the book? The ability of film media to infuse kitschy 1950's-style special effects throughout the movie. Faux sparkle added to a character's brilliant smile or dewy eyes. The trio of "jingle girls" singing the praises of a new sandwich spread as they offer the TV audience a 5ft. long sandwich through the screen. Evelyn Ryan zooming in outer space on her kitchen-appliance-contest-winnings. The campy effects made me feel that I was really watching TV in the 1950's, commercials and all. It adds a whole new dimension to the story when you see it on film.
Know of any other movies that turned out to be better than the book?