Many books have movies now, and most probably in the movie, not all the details of the story in the book will be the same.
Most book readers have recommended me to read the books before I watch it. In my case, I watch the movies before reading the books, so this decision depends on the reader; if you don`t like spoilers and would want to know the original story start by reading the book and then watching it.
I watch the movie first because it helps me to imagine things and understand details more clearly when I read the book, because the characters are already created for me, I don`t have to worry about what their wearing or what they look like, and also I like to make myself believe that the movie is the original story and in the books are the small details that you have to find which makes the story for much more interesting, exciting, and adventurous.
And also the movies don`t have time to include every scene in the book, so if there was a particular place where you really enjoyed in the book and it didn`t show in the movie you will be disappointed.