You've experienced just some of the .NET XML functionality. There are many other areas to explore still, but, as always, a lesson should be followed by some exercises to continue your learning. Here are a couple of things that could make the application better and further your knowledge in the .NET platform:
Make contacts searchable. (Hint: Use the keywords attribute with System.Text.RegularExpresssions.)
Change the namespace so that it can digest the XML through HTTP or the local file system.
Modify the namespace to better handle multiple addresses and phone numbers, regardless of the type attribute.
Build a lazily evaluated cache of contact instances into the ContactManager class.
Apply sorts on other attributes.
All of these not only delve further into the .NET's XML offerings, but all help strengthen an understanding of getting the most from .NET. In the next article on XML and .NET editing features, will be added to the namespace to dig further into the System.Xml namespace.