Don't be intimidated by the size of this book: You can use MH, xmh, exmh or MH-E right away by reading the quick-start tutorials. The rest of the book is here to help you do much more, but you don't have to read it all!
If you'd like to know many or all of the features of a particular system, read the part of the book that applies; use the table of contents or the index to find the part you want, and search for key words or titles with your browser's "search" command. Because MH-E, xmh, and exmh are interfaces to MH, you'll get a better understanding of a system by reading about the parts of MH that apply to it. Special tables of contents point you to all MH book sections (not just the main Parts) that apply to exmh and xmh.
Although this book has plenty of information that isn't in your online MH and xmh manual pages, those manual pages have some details that we've omitted from the book. The MH system has some 50 individual manual pages of its own; the mh(1) manual page summarizes them. The exmh online documentation is related to its part of this book, though the book has cross-references and figures that aren't in the documentation. The Reference List points to Internet standards and other books that will build a framework to help you get the most from your email system.