General UNIX

Hacking tips for UNIX	A description of a few small hacks
Improving the Security of Your Site By Breaking Into It	A very detailed guide for admins that outlines several common security holes and methods hackers use to break in.
A UNIX Hacking Tutorial	One of the best UNIX tutorials out there, goes from the basics up- very complete and excellent for the beginner.
COPS & Robbers-UNIX System Security	An outline of analyzing a UNIX system's security with the COPS program.
UNIX Security Checklist	Outlines common errors in administration and general security guidelines.
Unix System V	How to hack UNIX System V
UNIX Backdoors	How to hold on to root.
Hacking UNIX	Basic tutorial of UNIX security.
Bourne Programming	A thorough shell programming manual.
Architectural Overview of UNIX Network Security	An excellent guide to the layered security of networked systems.

Program Specific

Crash Course in X Windows Security	Explains the security risks of X Windows including screen capturing.
The SMTP Protocol	A complete description of the Simple Mail Transfer Protocol
Sendmail 8.6.4 Bug	Security hole that allows user to get root shell
NFS Tracing By Passive Network Monitoring	Spying on ethernet traffic to gain information on NFS filesystem activity.
Crontab Exploit	Takes advantage of a buffer overflow problem.
Internet Email Security	A good discussion of sendmail and some of its basic flaws.
Smashing the Stack for Fun and Profit	Excellent article explaining buffer overflows.

Other Operating Systems

Vax/VMS basics	A FAQ for new users of Vax/VMS systems, good for beginners.
Hacking VAX	Informative file on security protocols and measures on VMS.
The HP3000	Information on hacking Hewlett Packard's HP3000 mainframes.

TCP/IP and Misc. Networking

How Kevin Mitnick hacked Tsutomu Shimomura with an IP sequence attack	Very interesting, requires a knowledge of TCP/IP to understand.
Source Routing	An explanation of source-routing in conjunction with an IP spoofing attack and other situations.
IP Addressing	Information on the format of IP addresses, interesting but not for newbies.
TCP/IP Weaknesses	Flaws in BSD's protocol software.
Simple Active Attack Against TCP/IP	Rerouting TCP data streams to bypass authentication protocols such as kerberos.
DHCP FAQ	A thorough guide to the Dynamic Host Configuration Protocol
Ethernet FAQ	FAQ of Ethernet terminology.
The Novell Netware Hack-FAQ	A good resource on basic Netware security.

Packet Switched Networks

Tymnet Hacking	Brief notes on hacking the Tymnet network .
Telenet: The Secret Exposed	A very thorough guide to using/hacking Telenet.
Packet Switched Networks	A guide to Sprintnet and similar networks

Hacking Literature

Mentor's Last Words	Powerful words of wisdom, just read this.
The Hacker Test	Funny (if you understand it) and educational.
Diary of a Hacker	First person account, very interesting.
Diary of a Hacker II	The second part of the diary.
Concerning Hackers Who Break into Computer Systems	Interesting analysis of the hacker culture presented at the 13th National Computer Security Conference
Becoming an an Uebercracker	Not a serious guide, this is simply hilarious. Just read it and see for yourself.

Miscellaneous

How to Find Security Holes	Concepts in finding security breaches, mainly in software code.
Social Engineering	Exploiting human weakness-tricking people into revealing sensitive information.
Hole List	A lengthy compilation of security holes.
Denial Of Service	Unethical but I included it for informational purposes, it explains how to flood services, crash servers, etc.
Hacking Techniques	A VERY basic text file describing a few simple tricks.
The Internet Worm	Explains exactly what happened and how it worked.
The Risks of Key Recovery, Key Escrow, and Trusted Third-Party Encryption	An excellent article on modern cryptography.
Getting Information	Some methods of gaining information about people.
Cracking the UPC	Explanation of the barcoding system.