Generally movie helps promote the book. if the movie is good, sure it will encourage people to read the book (my case - Girl with the dragon tattoo, TLOTR...) usually the storyline will be a bit different and in most case, books are much better than the movie...

However, if the movie is badly made or just plain boring, people might lost interest or just tends to stay away from it when they saw it in the bookshop (my case - countless)