The 13th Warrior came from the book Eaters of the Dead. Good read, movie was so-so.
Hitchhikers Guide to the Galaxy, wtf were they thinking with the movie!? Yes, it had it's book moments but..I really cant put my disappointment into words. Now, the 1981 version of Hitchhikers,, that was good.
Add the Anne Rice vampire books that were destroyed into movies to this list too.