Network Troubleshooting Tools (O'Reilly System Administration)
Hardening Cisco Routers (O'Reilly Networking)
IP Routing
Cisco IOS Cookbook: Field-Tested Solutions to Cisco Router Problems (Cookbooks (O'Reilly))
Cisco Access Lists Field Guide
Network Warrior: Everything You Need to Know That Wasn't on the CCNA Exam
CCNA Security (210-260) Portable Command Guide (2nd Edition)
TCP/IP Network Administration (3rd Edition; O'Reilly Networking)