It takes place concurrently with Eclipse (which debuts in theaters June 30th). I am counting down the days! Here's what the website says:
There are two sides to every story...
Fans of The Twilight Saga will be enthralled by this riveting story of Bree Tanner, a character first introduced in Eclipse, and the newborn vampire world she inhabits. In another irresistible combination of danger, mystery, and romance, Stephenie Meyer tells the devastating story of Bree and the newborn army as they prepare to close in on Bella Swan and the Cullens, following their encounter to its unforgettable conclusion.
On June 5th visit http://www.breetanner.com/ to download your copy! It comes out close to my due date. Maybe I can download it and read it at the hospital (after I get my epidural)! Ha ha!