It's a pretty good indication that the book sucked, if the movie was better.
And it's odd to have a bad book become famous or beloved enough to launch a movie.
So bad books might be easier to film well than good ones - they're simpler, and you can cut still more without losing much, and rewriting all the dialogue doesn't do any damage - nobody misses it. Kind of like bad poetry being easier to translate ?
Ludlum (the Bourne series) is a miserable writer - I've never been able to figure out how he sold so many books. The first movie dropped most of his plot junk, changed the emphasis to something with a little depth, took advantage of the scenery, and improved the dialogue. What's not to like?
"Jaws" was a better movie than book.
"The Lost World"
"The Bridges of Madison County"