This article is excerpted from The Official Ubuntu Book and they take a look at filesystem security, system resource limits, dealing with logs and finally some network security.
Read it at Computerworld
As a system administrator, one of your chief tasks is dealing with server security. If your server is connected to the Internet, for security purposes, it's in a war zone. If it's only an internal server, you still need to deal with (accidentally) malicious users, disgruntled employees and the guy in accounting who really wants to read the boss's secretary's e-mail.