Further Reading

http://www.snort.org
mailing list / man page / code  
http://www.snort.org/docs/writing_rules/
http://www.securityfocus.com/ids

Book
Network Intrusion Detection, 3rd Ed - Judy, Northcutt
Snort 2.1 Intrusion Detection, Second Edition by Brian Caswell, Mike Poor, Andrew Baker, Toby Kohlenberg
Tao of Network Security Monitoring, The: Beyond Intrusion Detection by Richard Bejtlich

News
http://www.wormblog.com/ - a blog about the worm problem, technology, detection, and defenses
http://id.icious.net/blog/ - IDS Blog
http://id.icious.net/ - IDS Daily