This second installment of The Lord of the Rings is broken up into two parts: the first two-thirds follows Aragorn, Legolas, and Gimli as they seek out the hobbits Merry and Pippin, who star in their own sections. The last third follows Frodo and Sam as they make their way towards Mordor. I preferred the former, as the tale of Frodo and Sam's trek is gloomy and grave. Merry and Pippin at least are cheerful and wondrous, especially once they meet the Ents, and Aragorn's crew are noble and even a bit humorous at times.
Again, it's interesting to see how Peter Jackson translated the book into film and to note what liberties he took with the plot structure. For example, the battle for Helm's Deep takes up a small section in the middle of the book, whereas it is the massive climax of the movie. Another change of note is how much more intelligent Gollum is in the book. To be sure, he is insane and twisted, but he also seems to understand more and is much better at communicating. My guess is the changes Jackson made to his character were meant to make Gollum all the more loathsome. But I think we lose the sense that Gollum used to be Smeagol, and that he was once a creature much like Sam and Frodo, a point I think Tolkien wanted the reader to remember.