I read part of Real World Linux Security this weekend. It's a very detailed book that covers a wide range of security topics, from an author with lots of security experience. Some of the topics include adaptive firewalls (that log and block out intruders), how to be prepared in the event that a breach does occur, how to find Trojan's that may have been left after an attack, how to setup a fairly secure online ordering system that stores credit card numbers, and so on. From Amazon:

A hands-on guide to protecting Linux data from security risks. Introduces readers to the seven deadly sins of Linux security, showing how to set up firewalls, break in case studies, block spam, develop internal security policies, and recover from an intrusion quickly.