Burning Crusade Sourcebook will be the third unofficial fanbook to be released via free PDFs, after Even More Magic and Mayhem and Monster Guide II. The book is NOT an official book, White Wolf has nothing to do with it and the book won't be commercially available, they'll be released for free. The writers will try to include as much updated info they can. However, they don't have Blizzard's support on the project, so it won't bring anything new that isn't already known.
MagusRogue (Adam Lloyd) is involved in the project.
It was previously hinted at when Monster Guide II was announced. News of the book was accidently leaked on May 25, 2008.