If you want to start, or become an expert on Windows Phone 7 game development using XNA, I´d highly recommend you to buy a.s.a.p the following book:
http://www.amazon.com/Professional-Windows-Phone-Game-Development/dp/0470922443
It´s written by two masters of XNA and fellow MVPs Chris Williams and George Clingerman, and they did a really really good job.
What are you waiting for? Go get it !!