For the differences specifically all I did was read the book alongside the Peter Jackson film adaptation (Extended Editions). I'd read a page or so, then watch the film and see when it caught up to that point in the book. During all of this I used my references (listed on the main film v book page) to make sure it was actually a difference or not (whether moved to a different part of the film or cut entirely) or to make sure I hadn't missed anything.