Network Security issues

I currently do some network consulting in addition to my full-time job. I am interested in learning more about NETWORK SECURITY APPRAISAL/CONSULTING and looking into this as a service to offer my larger current clients an new clients.

My question is…Where can I learn more about network SECURITY (pitfalls to avoid, suggestions to follow, etc.) so I can better serve my clients with the security aspects?

Does anyone know of a SPECIFIC source I can go to to learn alot more about this topic (network security) and how to approach relevant network issues that will arise?

THANKS!