Cisco IOS in a Nutshell  
   O'Reilly & Associates guide to configuring for TCP/IP protocol family, user interface, configuring lines and interfaces, access lists, routing protocols, dial-on-demand routing, security, quick reference to commands to work with TCP/IP and lower-level
   http://www.oreilly.com/catalog/cisiosnut/

Cisco IOS: Internetworking Operating System  
   Root page for Cisco's network OS: sophisticated, cost-effective, dynamically upgradable, internetwork optimized, router-centric; provides common IP fabric, functionality and command-line interface (CLI) across a network.
   http://www.cisco.com/warp/public/732/

Cisco IOS Operating System  
   Brief, clear description, by OS and network consultants, Frigate Networks.
   http://www.frigate.com/cisco.html

Cisco IOS  
   Brief, clear definition and description, by Romanian Cisco Partner, Datanet Systems.
   http://www.datanets.ro/