Came across this on the MSDN blogs, was posted a couple of days ago. If you're interested in the basics of writing applications for Windows Phone 7 using the C# programming language with the Silverlight and XNA 2D frameworks. The EBook is free but the book will be sold split into 2 hardbacks as well.

In particular, you might want to buy other books to supplement the material in this book. For example, I barely mention Web services in this book, and thatís a serious deficiency because Web services are likely to become increasingly important in Windows Phone 7 applications. My coverage of XNA is limited to 2D graphics and while I hope to add several 3D chapters in the next edition of this book, I donít really get into the whole Xbox LIVE community aspect of game development. Nor do I discuss any programming tools beyond Visual Studioónot even Expression Blend.

This book is divided into three parts. The first part discusses basic concepts of Windows Phone 7 programming using example programs that target both Silverlight and the XNA framework. It is likely that many Windows Phone 7 developers will choose either one platform or the other, but I think itís important for all developers who have at least a little knowledge of the alternative to their chosen path.
The second part of this book focuses entirely on Silverlight, and the third part on XNA 2D. For your convenience, the chapters in each part build upon previous knowledge in a progressive tutorial narrative, and hence are intended to be read sequentially.
To use this book properly youíll need to download and install the Windows Phone Developer Tools, which includes Visual Studio 2010 Express for Windows Phone, XNA Game Studio 4.0, and an on-screen Windows Phone Emulator to test your programs in the absence of an actual device. Get the latest information and downloads at
PDF Download:BTW, you can download a PDF here (38.6 MB).
Code samples (C#): You can download the ebookís C# sample code here (5.03 MB).
Code samples (VB.Net): And you can download the sample code in Visual Basic for Chapters 1-18 here (5.21 MB).

Info from the authors blog here.