This book is too basic. It's real audience is people with little or no previous experience with mail servers.
I have long experience with Postfix and expected two things from this book:
a) Show me a holistic vision of Postfix to better understand how it works. Postfix docs are to terse and hard to integrate and interpretate. For example, while there is only one (main.cf) configuration file (with no subsections), there is no single doc that show all posible parameters and theirs meanings. Instead, each Postfix's component have their own documentation with their own parameter set explained. Postfix need very badly a table showing which parameters goes with which component, and this book doesn't provide one (and I expected one). The Postfix organization and operation is VERY logic after the fact, but it will be a lot easier if someone show you (and save you from making the same mistakes).
b) Explain how to do some real world tasks (for example, how to place differents recipient restrictions for inbound mail and for outbound mail).
This book adds nothing to Postfix's own docs; it's only a gentle introduction to the subject.