The set of documents that comes with MH 6.8.3 and nmh-1.0 (the latest versions, as of this writing) is more complete than what you get with a lot of programs in the public domain. But the documentation is big and is organized by command name, not by task. The tutorials in it are okay for getting started, but harder to use if you want to learn more than the basics. The xmh manual pages are fairly complete but don't have many examples; they also assume that you have other X documentation.
This book summarizes what I've learned in 18 years of using MH as a user, an instructor, a programmer, and a system administrator. It has the basics you'll need to get started. Then, it leads you through more advanced features, topic by topic. Here's where you'll learn the real power in this mail system. I'll explain the documented and undocumented features, point out some of the bugs, show you how to configure MH (MH is very configurable), and share tricks that I've learned for doing things faster and better.
The chapters on MH-E are written by a long-time user and the maintainer of the comp.mail.mh FAQ, the "Frequently Asked Questions" document. The author of exmh wrote the chapters about it. Although their chapters are similar to online documentation, this book adds cross-references and related information that isn't available online.