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
ISBN: 1-56592-124-0
Practical UNIX & Internet Security, 2nd Edition By Simson Garfinkel & Gene Spafford
2nd Edition April 1996
ISBN: 1-56592-148-8
Computer Security Basics By Deborah Russell & G.T. Gangemi, Sr.
1st Edition July 1991
ISBN: 0-937175-71-4
Linux Network Administrator's Guide By Olaf Kirch
1st Edition January 1995
ISBN: 1-56592-087-2
PGP: Pretty Good Privacy By Simson Garfinkel
1st Edition December 1994
ISBN: 1-56592-098-8
Computer Crime A Crimefighter's Handbook By David Icove, Karl Seger & William VonStorch (Consulting Editor Eugene H. Spafford)
1st Edition August 1995
ISBN: 1-56592-086-4
Linux Security By John S. Flowers
New Riders;
ISBN: 0735700354
March 1999
Maximum Linux Security : A Hacker's Guide to Protecting Your Linux Server and Network
Anonymous
Paperback - 829 pages
Sams;
ISBN: 0672313413
July 1999
Intrusion Detection By Terry Escamilla
Paperback - 416 pages (September 1998)
John Wiley and Sons;
ISBN: 0471290009
Fighting Computer Crime
Donn Parker
Paperback - 526 pages (September 1998)
John Wiley and Sons;
ISBN: 0471163783