What are recommended reading for developing Windows Phone apps?
I would definitely start with Channel 9 videos. Videos are always better than reading to start with in my opinion.
Here are several links that can help you get started.
Windows Phone 7 Development for Absolute Beginners: http://channel9.msdn.com/Series/Windows-Phone-7-Development-for-Absolute-Beginners
Free Book and best one to start with: “Programming Windows Phone 7” available at http://www.charlespetzold.com/phone/
“Learning Resources” section from http://msdn.microsoft.com/en-us/library/ff402535(VS.92).aspx
And then practice writing apps with
101 Windows Phone 7 Apps
Oh and do not forget the “MS Patterns and Practices Windows phone Developer guide” at http://www.amazon.com/Windows-Phone-Developer-Guide-applications/dp/0735656096/ref=sr_1_45?ie=UTF8&qid=1309816346&sr=8-45