You could apply the concepts and ideas from this book to other areas and continue to experiment and explore their effect in these new areas. For example, we've learned about Adorner objects, so you could use that new-found knowledge to implement some visual feedback for the common drag and drop functionality in the main window's adorner layer.
You could then further extend this idea, using what you've discovered about Attached Properties, and completely encapsulate this drag and drop functionality, enabling the developers that utilize your application framework to make use of this feature in a property-based manner.
For example, you could create a DragDropProperties class that declared Attached Properties, such as IsDragSource, IsDragTarget, DragEffects, DragDropType, and DropCommand, and could be extended by your relevant Attached Property classes, such as a ListBoxProperties class.
You could then declare a BaseDragDropManager...