We had a hard time getting some actual information about this book, but did find that Amazon.co.uk has it listed with some basic information.
The prequel paperback: Gears of War Pendulum Wars the Battle of Aspho Fields is written by Steve L. Kent, is 352 pages long, and published by Del Ray Books.
We've seen varying reports of the release date for the book. Amazon.co.uk displays a February 26 2008, but Wal-Mart has it listed as August 2008.
Either way, we're looking forward to learning a little more about our favorite COGs. Hopefully the book will detail the events surrounding Marcus's imprisonment and death of his father.