The relationship between a book and its adaptation is not a simple hierarchy where one is prior and the other is derivative. An adaptation is a reading — an interpretation made by the people who created it. Sometimes that reading discovers things in the source that the source hadn't fully developed.

The Shining is a better film than it is a novel. This is the example people resist most, because King himself dislikes the film. But Kubrick found something in the material — the hotel as a container for a man's capacity for violence rather than as a supernatural entity — that is more interesting than King's original horror story. The film is a divergent reading, not a faithful translation. As a reading, it's brilliant.

The Dune films are faithful in a different way. Villeneuve is less interested in diverging than in finding the cinematic equivalent of what Herbert was doing literarily. The result is more respectful of the source and also, to my eye, somewhat less daring as a standalone work.