Nokia make some fine APIs for Windows Phone. And now they've just released a couple more. Their Imaging API is great, and make it really easy to add image processing to your applications. It has just graduated from Beta to a formal release.
The Sensor Core API, just released as a beta, is a very interesting development. It takes Windows Phone into a domain it hasn't been before. Up until Sensor Core it was impossible to do things like track user activity and do things like count the number of steps walked by the phone user, find out where they had been etc etc. But Sensor Core provides the stuff to do all that for you.
From my experience Nokia libraries are well written and beautifully documented. I'm going to get hold of them and have a play. If you are into Windows Phone, you should too.