A Q&A article by Tolkien Scholar Michael Martinez
Here is a very interesting article that discusses the pros and cons of the fidelity (or lack thereof) book based movies have towards the book. Michael Martinez notes the difficulty of making a movie faithful to the book, especially Tolkien books, where main characters commonly die, and rights to much of Middle-earth's history is out of reach to Peter Jackson. He concludes by saying that Peter Jackson's changes are overall beneficial, if only because the characters are fleshed out and we learn more about them.
Source: Middle-earth & J.R.R. Tolkien Blog by Michael Martinez