If the first book introduced Edward, and the second book introduced Jacob, this is the book where Bella's internal struggle is played out, where she is forced to choose between the two. As if this wasn't enough, the past is starting to catch up with her. Introduced in the previous book, the vampire Victoria and the ancient Volturi coven are out to get Bella, and their paths is intertwined in a rather odd way.
More oddities happen when the truce between werewolves and vampires is strengthened, in fact turning into a joint effort to ensure the safety of Forks and the safety of Isabella Swan.
We are also introduced to more of the history of werewolves, as well as the history of werewolves. All this gets Bella torn, not being able to choose. However, she finally makes her choice in the very end, a choice that causes her much grief.
I'd say this book is mostly acting like a bridge tying up loose ends. It is, like the other books in the series, easy to read, and while there is not much happening in terms of concrete action, the events taking place in Bellas mind are enough to cause both laughter and tears. It's easy to sympathize with the choices she has to make and feel for her.
The book ends in such a way that it's almost impossible to resist opening the first page of Breaking Dawn, the final book in the Twilight Saga.