the qmail handbook
If you've ever been serious about using qmail then you've read Life With qmail. Sure, there's a ton of other qmail tutorials, but LWQ has become the de facto standard for qmail. The same author, Dave Sill, took LWQ and put it in a more detailed, book oriented form and called it The qmail Handbook.
It's quite normal nowadays for a book to come out touting the title 'Handbook' but being nowhere close to a definitive guide. This book, however, is. I started this book barely grasping the concepts of an email server but came out being able to maintain a stable, secure, and featureful mail server. It covers everything from how email works, to how daemontools work, to how qmail works, gluing it all together, then showing you how to start adding features to it like spam control and remote access.
Though this book is almost four years old, Dave has written an amazing book that will still, to this day, teach you how to put a rock-solid mail server together.
This book gets a 10/10.
