If you are curious about developing your first Windows Phone 7 application, either using Silverlight or XNA, Charles Petzold has published a free preview of his “Programming Windows Phone 7 Series” book due for release in the fall of 2010. The free eBook contains 6 chapters (150 ish) pages walking you through the basics of programming with Silverlight and XNA for the phone.
For the book he assume that you know the basic principles of .NET programming and you have a working familiarity with the C# programming language. If not, you might benefit from reading his free online book .NET Book Zero: What the C or C++ Programmer Needs to Know about C# and the .NET Framework, available from www.charlespetzold.com/dotnet.
Download a copy in either PDF or XPS of the book from http://www.charlespetzold.com/phone/index.html. You can also download sample source code to easier get you started with your development.
It’s worth noting that it’s not just the preview that is free. The complete book will be free when it is finished too.