Microsoft is offering Free ebook which covers basics of writing applications for Windows Phone 7 using the C# programming language with the Silverlight and XNA 2D frameworks.
Book is about 1000 pages with 24 chapters and well, its a gift from Microsoft team to developer community. Getting a book written by Charles Petzold through Microsoft Press is one of the best thing to happen for a developer.
You can download a PDF here (38.6 MB).
And you can download the ebook’s sample code here (5.03 MB).