Entitled “Lazy Linux: 10 essential tricks for admins“, this is a pretty good set of essential commands for *nix administrators.
If I have to suggest anything for in-depth study, it’s “Trick 6”: “Remote VNC session through an SSH tunnel”, because you can actually do just about anything through an SSH tunnel. If you’re faced with you -> firewall -> internet -> remotemachine -> internet -> restoftheworld, you can do whatever you like with “restof theworld” if you can ssh to “remotemachine”, whatever the local firewall thinks. And you can probably ssh over port 443 through the firewall. Hmm; maybe I should write this up properly at some point. I’m sure it’s well documented on the web if you look for it….