I recently came across a couple good security-related articles that's well worth reading.
The first is an introduction to firewalls and backdoors, describing each type, listing examples, and providing tips. Also contains links to some very useful external resources. http://securityfocus.com/infocus/1701
The second article disusses real-time alerting with snort, the most popular open-source IDS (Intrusion Detection System). http://newsforge.com/article.pl?sid=03/06/09/1939256. It does not cover installation or initial configuration, however; check out the snort documentation for details on this.