I’ve already bought this book, which for $10 is a bargain. I’m only up to chapter 3 so far but I would say, this book could be for more than just Windows Metro developers. It is a very good introduction to XAML, via C#, so almost all of it so far is applicable to Silverlight, WPF and Windows Phone development. I suspect later chapters will be more specific to WinRT.
Buy the book here: http://shop.oreilly.com/product/0790145369079.do