Part 13 from Andrea Boschin's article series on Windows 8 Metro is already published at SilverlightShow.
From the author:
There are many analogies between Windows Phone and Windows 8 metro-style applications, and the Application Bars are for sure one of these. Windows Phone introduced the application bar in his first release and, no matter if it's rough and limited, it's an important surface that adds space to the the main phone canvas. Windows 8 reprises and extends the application bar's concept making it so much flexible and extensible that requires a specific guidelines to rule the ways it may be used.
NOTE: The whole series will gradually be published into a multi-part ebook. Part 1, including the first 7 chapters from the series, is already available on SilverlightShow Ebook Shelf.