I read this because I had a copy of it on my Kindle and I had just read the rest of the Twilight series. I didn't thnk much of it, I thought it was mostly a money-making endeavour. It wasn't even a particularly interesting character to expand on. I think the first few chapters I read of Midnight Sun (Twilight told from Edward's perspective) were much more enthralling.
Bree can barely remember her human life. All she knows is her thirst and the newborns she has grown up with. She befriends Diego and he informs here that vampires can go in sunlight without being harmed, despite what their leader told them. They begin to question their nameless and faceless creator. They are sent to destroy a group of 'rogue' vampires (The Cullens) with all of their newborn strength. Bree does not believe what they're doing is right and doesn't join in. Carlisle lets her join them, despite the fact she is very tempted by Bella. When the Volturi finally show up, once all the newborns are already killed, they also kill Bree because she is a liability.