There are a LOT of good sites out there for Unix security in general and Linux security specifically. It's very important to subscribe to one (or more) of the security mailing lists and keep current on security fixes. Most of these lists are very low volume, and very informative.
CERT is the Computer Emergency Response Team. They often send out alerts of current attacks and fixes. See ftp://ftp.cert.org for more information.
ZEDZ (formerly Replay) ( http://www.zedz.net) has archives of many security programs. Since they are outside the US, they don't need to obey US crypto restrictions.
Matt Blaze is the author of CFS and a great security advocate. Matt's archive is available at ftp://ftp.research.att.com/pub/mab
tue.nl is a great security FTP site in the Netherlands. ftp.win.tue.nl
Bugtraq: To subscribe to bugtraq, send mail to email@example.com containing the message body subscribe bugtraq. (see links above for archives).
CIAC: Send e-mail to firstname.lastname@example.org. In the BODY (not subject) of the message put (either or both):
Red Hat has a number of mailing lists, the most important of which is the redhat-announce list. You can read about security (and other) fixes as soon as they come out. Send email to email@example.com with the Subject
SubscribeSee http://www.redhat.com/mailing-lists/redhat-announce-list/ for more info and archives.
The Debian project has a security mailing list that covers their security fixes. See http://www.debian.com/security/ for more information.
There are a number of good security books out there. This section lists a few of them. In addition to the security specific books, security is covered in a number of other books on system administration.
Building Internet Firewalls By D. Brent Chapman & Elizabeth D. Zwicky
1st Edition September 1995
Practical UNIX & Internet Security, 2nd Edition By Simson Garfinkel & Gene Spafford
2nd Edition April 1996
Computer Security Basics By Deborah Russell & G.T. Gangemi, Sr.
1st Edition July 1991
Linux Network Administrator's Guide By Olaf Kirch
1st Edition January 1995
PGP: Pretty Good Privacy By Simson Garfinkel
1st Edition December 1994
Computer Crime A Crimefighter's Handbook By David Icove, Karl Seger & William VonStorch (Consulting Editor Eugene H. Spafford)
1st Edition August 1995
Linux Security By John S. Flowers
Maximum Linux Security : A Hacker's Guide to Protecting Your Linux Server and Network
Paperback - 829 pages
Intrusion Detection By Terry Escamilla
Paperback - 416 pages (September 1998)
John Wiley and Sons;
Fighting Computer Crime
Paperback - 526 pages (September 1998)
John Wiley and Sons;