Rumours have been pointing to Microsoft announcing Forza Motorsport 3 at E3 next month, and just in case you wanted more evidence that a third game in the hit racing series is indeed incoming, look no further than a listing on Amazon for the Forza Motorsport 3: Prima's Official Game Guide.
The guide is listed for release on September 30 and is said to contain 192 pages. Other than a couple of International Standard Book Numbers there's no more info listed. A quick Google of the ISBNs reveals the book is listed at a number of other online retailers, although some have already been removed.
The guide can also currently be found on the Prima website by directly entering the ISBN into the URL. The website states the guide will be released on September 8.
If the guide is hitting stores in September, it's safe to assume the game will be available in that month.