If you can't hack it, you can't secure it. We have this expression in the Dutch language: the worst poachers become the best foresters. I'm making my living now with securing networks, all knowledge I would have learned if I wouldn't have found anything on the 'net. Actually, I own zero books on Unix, Posix or system administration, and didn't pay for any of my software, so I'm very grateful for whatever knowledge is shared online.

Besides, the cat is out of the bag a long time already; look for phrack on your search engine of choice - excellent learning resource.