When translating a book into a movie I think it is much more important that the spirit of the book is translated than the actual literal story. Right now I am reading the book version of the new Scarlett Johansson film Under the Skin. Although the very basics of the story are similar so far, a female alien in the guise of a human drives around through Scotland picking up hitchhikers, much of it is different. However, they both create a similar mood. (And I admit that I am only partially through the book.) There is a sort of contemplative melancholy to both, although both occasionally feature very subtle dry humor.
In the book the main character is sort of bizarre looking, aside from her large breasts that keep being mentioned. In the film Scarlett Johansson is a femme fatale whose beauty lures men to their doom. There are also differences in the story itself. In the movie she takes the men to a house and in the book it is a farm. However, again the emotional feel of both is very similar. In that way I feel the movie is true to the book while being something unique and worthwhile in its own right.
Meanwhile, despite I know a lot of people loving it; I did not enjoy the movie No Country for Old Men. Had I never read the book I feel that I might have. However, I felt the movie, while being a faithful adaptation in terms of story and character, was simply a visual retelling of the book without the inner dialogue that made the book so fascinating. It was too literal of an adaptation. But that emotional truth, the kind that is represented by the inner thoughts of the characters, seemed lacking to me.
There is not necessarily a right way and a wrong way to adapt books to film. However, I definitely lean towards the idea that it is much more important to get the emotional content of an adaptation right than to literally retell the story. Movies can never be books. However, I am satisfied if I walk out of a movie feeling the same way I did after reading something I liked, whether or not the story is the same.