It doesn't have a release date but I chatted with one of the co-authors on her wall and she said that its final stages and will be out sometime in the fall or winter. The book is 404 pages and seems to be a must buy for all Lost fans. I've been looking forward to this since I found out about it a few months ago.
http://www.amazon.com/Lost-Encyclop...=sr_1_1?s=books&ie=UTF8&qid=1282350808&sr=1-1
http://www.amazon.com/Lost-Encyclop...=sr_1_1?s=books&ie=UTF8&qid=1282350808&sr=1-1