The CERT Coordination Center. Security advisories about holes in operating systems and software.
The Network Administrator's Guide. Describes TCP/IP networking, routing, DNS, configuring network devices in Linux, PPP & SLIP, NFS, setting up email.
The Linux System Administrator's Guide. Some good introductory material on systems administration on a Linux box.
The Linux Firewall Howto. Teaches the basics of firewall systems and give you some detail on setting up both a filtering and proxy firewall on a Linux based PC.
The IPCHAINS How-To. Describes how to obtain, install and configure the enhanced IP firewalling chains software for Linux, and some ideas on how you might use them.
The Shadow Password Howto. Shadow password files are good for you!
The IP-Masquerade Mini-Howto. Describes how to enable IP masquerade feature on a Linux host, allowing connected computers that do not have registered Internet IP addresses to connect to the Internet through your Linux box.
Security Focus. Lots of good Unix/Linux security resources there. Bugtraq mailing list archive is there also.
RootShell. The place to go for exploits and holes on your operating system.
COAST Hotlist. Links to computer security, law, and privacy sites.