Microsoft Press just made this draft version of the Programming Windows Phone 7 Series ebook from Charles Petzold available as a preview download.
More Details:
http://blogs.msdn.com/microsoft_press/archive/2010/03/15/free-ebook-programming-windows-phone-7-series-draft-preview.aspx
Direct Download Location of eBook: http://download.microsoft.com/download/7/C/8/7C820C6F-C205-4ECF-B9F3-1505DD13F9BF/ProgWinPhonePreview.pdf