There are a bunch of resources on Windows Phone 8 development for free.
Windows Phone 8 Jump Start
Recently we have the Jump Start and the full content is available at Channel9. Here is the list
In fact if you go in any of the links it would have reference to all other materials.
The Windows Phone 8 Training Kit for Developers
Windows Phone 8 in Build Conference 2012
Another great resource was delivered in Build 2012 conference. The complete list is available in Phone blog and here are they