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)

Download the pdf here: The 60 minute network security guide