So Bree Tanner made a brief appearance in Eclipse, book three of the Twilight Saga. We know her name, but why? Why do we know the name of a girl that appeared for less than 3 pages? Because Stephenie Meyer had something more for her in mind.
In The Short Second Life of Bree Tanner, we learn a little bit about the girl behind the vampire. We also learn a little bit more about the epic fight scene starting the Cullen Clan, Jacob's werewolf tribe, and the Volturi. But where does Bree fit into the big picture? She's part of Victoria's army of newborns. Their leader, Riley, has convinced them that they will do ANYTHING for her. They will fight the "yellow-eyed vampires" and they will know which ones when they see the pet human they keep. This book was basically a story of lies, deceit, and angry. But mostly, just death.
Now I may be a little biased because, lets be real, Twihard speaking. But I think this book really was a nice addition to the Twilight Saga. However, we didn't really need it. I think if this book would have come out as an Eclipse 3.5 and is read before Breaking Dawn, it may be of more use.