Network Security Information: UNIX  
Network Security Information: UNIX
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Incident Handling Step by Step: Unix Trojan Programs(paper)
SANS guide.

Unix Security(paper)
Just an introduction. 

HP Unix Security Handbook(faq)
Good and not very old paper. 

Linux Administrator's Security Guide(book)
Very useful. Almost FAQ. 

How to bypass Solaris non-executable stack protection(paper)
Ops. I thought non-exec is a coolest protection. Actually, it is not :(

Basic SUN Security(paper)
Short paper, mostly about applying patches.

Securing Your Linux Box(paper)
Don't you know it? You have to secure your Linux! ;-))).

Adding Security to Common Linux Distributions(paper)
Nice and short guide.

Securing Linux, Part 2(paper)
Advanced Linux security

Securing Linux, Part 1(paper)
Elementary security for your Linux box

SGI security Frequently Asked Questions (FAQ)(faq)
Latest! But VERY old - 1997

FreeBSD Security How-To(faq)
Nice and alone description about BSD security

Playing with the Internet Daemons(paper)
Description of most know daemons and their ports

Setting up sendmail on a firewall(paper)
Security techniques part

A Short Course on Systems Administration and Security Under Unix(book)
Extremely rigth title - Short Course. Almost hand-book

Security holes manifest themselves in (broadly) four ways...(paper)
All about insecurities - were and how to check

The Ultimate Beginner's Guide To Hacking And Phreaking(paper)
I don't think here is much about security but you can find a list of UNIX-types and their password files locations

Placing Backdoors into a UNIX computer(paper)
One more guide about backdoors

Placing Backdoors Through Firewalls(paper)
How to set it and how to protect from it.

Anonymizing UNIX Systems(paper)
I do not know if you will use all those advices but some of them are helpful