If you want to do Windows Phone 7 development, then get this book: Programming Windows Phone 7 by Charles Petzod.
The eBook and source code are free:
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)
In addition to C# and VB.NET, you can also do this in Delphi Prism.
Cool times ahead, because now there are 4 major competitors (in no particular order):
- Windows Phone 7
I left out the competitors of the past, as they soon will be deprecated.