First steps toward a secure network environment. All in one PDF file! Created by National Security Agency.

Contents include:

Security policy

operating systems and applications: versions and updates

Know your network

TCP/UDP servers and services on the network

Passwords

do not run code from non-trusted sources

read e-mail as plain text

other malicious code countermeasures

follow the concept of least privilege

application auditing

network printers

simple network management protocol (SNMP)

network security testing

Also some in-depth guides:

Perimeter routers and firewalls

windows 2000 and above operating systems

microsoft web server

unix systems and networks

unix web servers

intrusion detection systems (IDS)