Packet Guide to Voice over IP: A system administrator's guide to VoIP technologies
Cisco IOS Cookbook: Field-Tested Solutions to Cisco Router Problems (Cookbooks (O'Reilly))
Network Troubleshooting Tools (O'Reilly System Administration)
Network Warrior: Everything You Need to Know That Wasn't on the CCNA Exam
Packet Guide to Routing and Switching: Exploring the Network Layer
Hardening Cisco Routers (O'Reilly Networking)
TCP/IP Network Administration (3rd Edition; O'Reilly Networking)
Guide to Wireless Communications