For me it depends whether it is a type of book I would read, because there are stories I would not read the book but I would see the movie. For instance I probably would never read The Shawshank Redemption but I love the movie. What I also find interesting is when there are more than one movie version of a book. Two examples are I Am Legend which has three movie versions (my favorite being the third with Will Smith) and The Girl With The Dragon Tattoo (The Swedish version is Technically more accurate to the book in parts, but it lack much of the flavour and character charm that the U.S. version has.
I think the issues come in when the director wants to use an already popular novel or series to make a name for themselves, and the only way they can do that is by showing how they altered the story, which often ruins it for people.
See with; the girl with the Dragon tattoo, I saw the movie first, than I went to read the book. Mind was blown; Was like watching the movie again. I did appreciate what they did with the American version however the Swedish versions was good, followed the book very well, but I felt low quality, they were all produced in the same year. Both American and Swedish version had there moments which dictated the books story and some moments were also lacking. But over all both theatrical elements fitted the book very well, or came very close to capturing the books story line..
I agree with "The Girl With The Dragon Tattoo (The Swedish version is Technically more accurate to the book in parts,
but it lack much of the flavour and character charm that the U.S. version has" I felt the characters were stunning and captured the emotion, raw, real characters that the book portrayed. The Swedish to me, felt a little rushed, and pressed for time. I could be wrong, it's been a fair while.
I have never read I am legend, but I heard will Smith's film was completely off from the book.
another movie that followed the book; (but the 1987 film lacked many) was flowers in the attic. Given the time; I'm sure no one wanted to see that on screen and I'm sure it was taboo back in the 70s and still is. The lifetime version captured the book. The lifetime movie is what implored me to read the book, switched on the 1987 film on Netflix after I finished and was disappointed on how bad they deviated from the story.