The patterns&practices team at Microsoft has just released an update to last year’s Windows Phone Developer Guide. With release of Windows Phone 7.5, and the Windows Phone 7.1 SDK, the team has updated the guide to “A Case Study for Building Advanced Windows Phone Applications: Connecting with Cloud Services".
This guidance is built for developers looking to developer high-quality software. It might be overkill if you are looking to develop a really simple Windows Phone application.
In addition to the “Case Study for Building Advanced Windows Phone Applications: Connecting with Cloud Services”, the team has also spent time on guidance around “Building Testable Windows Phone Applications” as well as a piece of guidance on the MVVM (Model-View-ViewModel): “Developing a Windows Phone Application using the MVVM Pattern”. There is full detailed documentation as well as sample code and libraries.