The papers on the SANS Institute's site.
The "Basics" tab at SecurityFocus.com.
Articles labeled "Foundations" in WatchGuard’s LiveSecurity archive. (WatchGuard customers only).
Robert Graham's Network Intrusion Detection Systems (NIDS) FAQ.
Rebecca Bace’s whitepaper, "An Introduction to Intrusion Detection and Assessment."
Bace, Rebecca, Intrusion Detection(Pearson Higher Education; ISBN: 1578701856).
Escamilla, Terry, Intrusion Detection: Network Security Beyond the Firewall (John Wiley & Sons; ISBN: 0471290009).
Smith, Richard, Internet Cryptography, Addison-Wesley Pub Co; ISBN: 0201924803
Schneier, Bruce, Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd Edition, John Wiley & Sons; ISBN: 0471117099. This detailed volume might be too much to handle in your "spare time," but if you have the need and the interest, it is thorough.
Singh, Simon, The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography, Anchor Books; ISBN: 0385495323. This book gives valuable background on the history that has led up to encryption, useful more for context than for direct practical application.
E-mail Security, Why Don't We Bother?
E-mail Security, Part 2: Speed Bumps
Think your e-mail is secure? Think again.
E-mail security -- Defending the server.
Corporate E-mail: What's Your Policy?
Remote E-mail Access
Other Solutions For Secure E-mail.
Signed, Sealed, Delivered.
Rose, Marshal T., and Strom, David, Internet Messaging: From the Desktop to the Enterprise, Prentice Hall; ISBN: 0139786104
The SANS Institute.
The Internet Security Conference Newsletter.
The SANS Newsletter.
SC Info Security News.
Information Security Magazine.
CSI (the Computer Security Institute), which also runs regional and on-site training.
The SANS Institute, which also runs regional training.
The Internet Security Conference, TISC.
The USENIX Security Symposium.