Books

Building Internet Firewalls, Second Edition, by Elizabeth D. Zwicky, Simon Cooper, and D. Brent Chapman. O'Reilly & Associates, Inc.

Intrusion Signatures and Analysis, by Stephen Northcutt, Mark Cooper, Matt Fearnow, and Karen Frederick. New Riders Publishing.

Network Intrusion Detection, An Analyst's Handbook, Third Edition, by Stephen Northcutt and Judy Novak. New Riders Publishing.

Practical Unix & Internet Security, by Simson Garfinkel, et al. O'Reilly & Associates, Inc.

TCP/IP Illustrated, Volume 1: The Protocols, by W. Richard Stevens. Addison-Wesley Professional.