It's very hard compressing a novel into a two hour movie. Sometimes it works well, and sometimes it doesn't. Yes, a movie can help book sales, and book sales will help the movie, but ... There are lots of factors coming into it, including the writer of it, who can turn out a good script or a bad one, and have anything that follows ruin the script or enhance--the director, the actors, the editors, the studio.
I read Sahara and saw the film. The film wasn't very good. For all the money spent and this lawsuit, it's a b movie. It's something you'd watch once, shrug, and never watch again. The team that made the movie just simply didn't do a good job of putting together the movie, and I don't think the actors worked well as the characters.