The first question people might ask about this book is, how does it compare with The qmail Handbook by Dave Sill (Publisher: Apress). Dave, of course, is the standard, starting with the famous Life with qmail, an online manual long considered the qmail bible. When he wrote his handbook, it also became the standard.
Having said that, I see no reason to dismiss John's book in the least. If anything, I would call it a complement to Dave's book. In part, much of the same basic information is available, and also answers to a lot of questions that have appeared on the mailing lists over the years.
The common information is presented in a different way, which may aid in understanding both generic email and qmail specific issues, if you do read both books.
I particularly appreciate that John addresses the issues of running multiple instances of qmail on a single server, and multiple computers working together, all running qmail. Very useful for those who may want to replace some very large systems. Last I heard, qmail was being used for an ISP in India that had 20,000,000 or more accounts - I'm sure they are using more than one server!
I'm definitely happy to have this book on my shelf, both as a reference and as a source for ideas.