Dent begins with some background on Postfix and its philosophy, and on email, SMTP and Unix more broadly. Two chapters then cover the basics in some thirty pages: one describes the Postfix architecture and another its configuration and administration. There follow chapters on management of the Postfix queue, basic DNS for email, local delivery — mbox and maildir formats and POP/IMAP options — hosting multiple domains, and mail relaying.
The remaining chapters cover more specialised topics. There's a chapter on running mailing lists, covering simple :include lists managed by Postfix itself, majordomo, and mailman. There's a chapter on Postfix's extensive built-in spam controls and another on interfacing to external content-filtering software. And there are chapters on SASL authentication, TLS, and using an external database such as MySQL or LDAP.
How useful this material is obviously depends on what you want to do. I wish I'd had the section on getting mailman to work with Postfix back when I did that. And I found the chapter on spam controls most helpful, using it to rewrite my anti-spam configuration completely. But I only glanced at the chapters on SASL, TSL, and databases.
The Definitive Guide provides examples, but not much in the way of step-by-step recipes — for those it's probably better to look online for one which matches your operating system and setup exactly. What Dent's book will give you is the background necessary to understand what's in the recipes, or to experiment yourself.