Here's part of what Stephenie Meyer says about Bree Tanner on her website:
"Actually, this has been a surprise to me, too. The reason why it's a surprise was that I never intended to publish this story as a stand-alone book. I began this story a long time ago—before Twilight was even released. Back then I was just editing Eclipse, and in the thick of my vampire world. I was thinking a lot about the newborns, imagining their side of the story, and one thing led to another. I started writing from Bree's perspective about those final days, and what it was like to be a newborn.
This story was something that I worked on off and on for a while, just for fun, in between the times I was writing or editing other Twilight novels. Later, when the concept for The Twilight Saga: The Official Guide came up, I thought that might be a good place for Bree. Her story is a nice complement to Eclipse; it explains a lot of the things that Bella never knew. So I dusted it off and finished it up for placement in the Guide."Can't wait! Visit http://www.stepheniemeyer.com/ for more info.