The Chapter Using xmh goes into detail about xmh features introduced here, as well as other features. It explains more about sequences, which provide a way to organize and keep track of your messages, and also covers other xmh features such as folders. You can change the way that xmh works, too -- the Chapter Customizing xmh explains how.
Here's a list of commonly-used features of xmh -- and a few others that are good to know, anyway.