The Return of the King was my favorite of the three Lord of the Rings movies, but out of the three books it's probably the most flawed. For one, a lot less happens than in the first and second books (the first particularly is just crazy wizard shit after crazy wizard shit). Most of the book is overshadowed by two big events: the Battle of the Pelennor Fields, in which the forces of Gondor and Rohan beat back the forces of Sauron's Mordor, and Frodo and Sam's final trip into Mordor to destroy the Ring. In retrospect, it's probably good that Peter Jackson was least faithful to this book out of all of them, because the movie is more exciting.
The other problem is that all that is finished by page 200 (out of 300). The final third of the book drags on excessively after most conflict has ended, plowing through funerals, and coronations, and lots of goodbyes and such. The only interesting part of the final third is the Battle of Bywater, in which the four hobbits amass a hobbit army in order to reclaim the Shire from Saruman, who has moved in and taken over since he was expelled from Isengard.
What I did like about this book, though, was the character of Denethor. Too many of Tolkien's characters are wholly good, noble characters--there is no discernible difference in character, for example, between Faramir and Eomer and Theoden and even Aragorn. But Denethor, King of Gondor, is a flawed character and thus more interesting: He is always suspicious of Gandalf, who he thinks wants to take over his kingdom, and then when his son Faramir is gravely wounded in battle, Denethor goes crazy and assumes he's dead and then tries to burn him (and himself) on a funeral pyre. What a nut!
Anyway, I'm both glad I read these books and glad I'm done reading them. Few other books give the same impression of having read something huge and important, but they can be tiring and the storytelling is not always fantastic. Far more interesting than the books themselves is all the stuff that went into their creation, like the languages (you can learn Quenya, or Elvish, today, and it's only one of tons of languages Tolkien created for the book) and the geography and history.
Here is a Middle-Earth map for nerds.