This book is a gift from the Windows Phone 7 team at Microsoft to the programming community, and I am proud to have been a part of it. Within the pages that follow, I show you the basics of writing applications for Windows Phone 7 using the C# programming language with the Silverlight and XNA 2D frameworks.
Yes, Programming Windows Phone 7 is truly a free download, but for those readers who still love paper—as I certainly do—this book will also be available (for sale) divided into two fully-indexed print editions: Microsoft Silverlight Programming for Windows Phone 7 and Microsoft XNA Framework Programming for Windows Phone 7. [Note from Devon: we should have these ready for order in December 2010.]
Full details at http://blogs.msdn.com/b/microsoft_press/archive/2010/10/28/free-ebook-programming-windows-phone-7-by-charles-petzold.aspx.