THE Sendmail book.
Over the years I've troubleshot and configured a few Sendmail servers, and this book is absolutely the be all end all of books on the subject. It holds a massive amount of information inside of its 1000+ pages, putting other guides to total shame.
Sendmail may be long in the tooth anymore, but if you ever plan to implement a new server for whatever reason, you will want this book by your side.
It will take you from the very basics of running a Sendmail server and all of the fundamental knowledge you'll need to get started, to incredibly advanced administration topics. And, to top it all off, the appendices of this book hold pretty much every piece of random Sendmail knowledge you never knew you wanted to know.
Overall it's an amazing book, and oldy but by all means a goody. Nothing will make you feel more like a Linux wizard than carrying around a copy of this massive tome.ee