Gubtitle: E-mail for users and programmers, Third edition
Author: Jerry Peek
Publisher: O'Reilly
Publication date: April 1995
ISBN: 1565920937
Pages: 782
Price: 34.95 US
WWW book information: http://www.ora.com/catalog/mh3/noframes.html
Examples: http://examples.oreilly.com/mh3
Besides the obvious, this book also covered the Tk application exmh. According to the publisher, this book has been discontinued.
However, one is likely to be able to find the book via liquidation or used book sale web sites, etc.