spamassassin
If you run an email server, configuring it to use SpamAssassin has become mandatory. In the past month, SpamAssassin has tagged 2334 messages as spam on my server. 1287 of those were directed towards me. That's about 2-3 times the amount of legit mail I receive.
Alistar does a great job covering SpamAssassin with this book. Though it's only about 200 pages, it has everything you need to know about spam and using SpamAssassin to prevent it. Everything from what spam is, installing & configuring SpamAssassin, using external services such as Razor and Pyzor, writing your own SpamAssassin rules, and a quick tutorial on setting up your own standalone SpamAssassin gateway is covered in this book. It's short, but it gets right to the point.
I give it a 9 out of 10.
