This book was origionally purchased for my by a boss and friend of mine for my use with a project, and I still reference whenever I'm stuck with a Postfix problem, or gearing up for a new installation.
I have to say that for any aspiring Postfix administrator, this is a must have. The explanations are vibrant and interesting, and the overall format of the book is wonderful.
I used this book from the perspective of never touching Postfix previous to picking it up, and within the first two chapters (about half an hours worth of reading) I had my first server set up on Slackware and ready to send and receive mail.
Past the initial setup and troubleshooting, the book can give anyone a solid grounding in the theory behind administering a mail server, as well as things like spam and virus filtering, and pretty much everything else under the sun in relation to mail.