Well, there's two ways to look at it:
(1) Do you want to secure your network and keep your life mostly free of hackers/etc.?
- or -
(2) Do you want to get up to the level so that you can go find a job doing newtork security administration?
(1) Yea, the big red Hacking books [Hacking Windows 2000 Exposed, Hacking Exposed, Hacking Linux Exposed] are excellent places to begin. I'd recommend checking out a few websites like
SecurityFocus to get a grip on the basics. Taking classes is probably a bit overkill and getting a certification definitely is.
(2) Certifications are your friend. First, start with the advice above if you know nothing - then aim for the CCSP certification (it's not stricly network security oriented, it's more "Cisco Network Security", but it's damned good to have - I'm working on mine). I'd say learn anything you can about firewalls, Instrusion Detection Systems, VPNs, etc. Let you brain be a sponge, and allow the internet to soak into it.
Above all else, no matter which route you choose: Experiment. Theoretical knowledge gained through book learning is effectively useless. Find an old PC at a yard sale, slap a copy of Linux or BSD on it, then find another old PC and slap a copy of Windows on it. Then try to hack the machines, try to secure them, try to hack them again - examine the log files, etc. Try to figure out what traces hackers would leave, how to keep an eye out, etc.
If you want to know more, let me know and I'll be happy to give you a listing of some of the good books I own.