You can get a free ebook, Programming Windows Phone 7, by Charles Petzold here: http://blogs.msdn.com/b/microsoft_press/archive/2010/10/28/free-ebook-programming-windows-phone-7-by-charles-petzold.aspx
Microsoft is offering free e-books from Microsoft Press. The subjects that I have seen, LINQ, ASP.Net AJAX, and Silverlight 1.0. All of these are introduction e-books. Those are new cool technologies. I would like to encourage you to take advantage of this free e-books here.