I currently make my living as a Linux Systems Administrator. This was one of the first books I ever read on the subject, and it still holds up pretty well. I think the edition first read was purple, I'm not sure what edition that was but this green one was published in 2002. It does still hold up pretty well, Linux really hasn't changed that much at it's core in the intervening years.
Some might think it weird that as a Linux admin I use primarily Macs as desktop computers, but really it's not. OS X has at its heart a Unix based kernel, and most of the pieces of the GNU toolkit are there, there really are few things that can't be done in OS X that can also be done in Linux. The main thing that I wrestle with in using OS X stems from the whole Open Source philosophy. But that's a way bigger subject than I feel like getting into here at the moment.