I bought this book for the procmail chapter (Chapter 3) but the other chapters, on sendmail and mailing lists, are also very useful. There is also an enlightening chapter on the history of email. The bulk of the book is a collection clear recipes along with explanatory text, in a practical how-to-get-tasks-done approach. In contrast to other encyclopedic books (in the case of sendmail) or disorganized masses of inconsistent web information (in the case of mailing lists), this book cuts to the chase and gives the key information you need.
Unfortunately, however, in the chapter I needed most (and the chapter I have scrutinized most), I found there are serious flaws. For example, the first two procmail recipes given under 3.5.2 on page 102 resulted in several days worth of my email getting deleted; they simply do not work as described on my system; when I removed these two recipes my email again worked fine.
If you are going to buy this book, wait for the author to put up an errata page, and then be sure to check that page before relying on the recipes in the book. In the meantime I still think the book has its merits since it explains things clearly.