|The stupid question is the question not asked|
It's a dirty job, but someone told me I had to do it
Another good book is the Unix System Administration Handbook, which has the nice feature of pointing out specificities of different versions of Unix along with the more general descriptions.
I would like to stress the advice about staying (starting now) up to date on the latest happenings, particularly in security: subscribe at least to BugTraq and to the security mailing lists of the vendors whose OS's you will be administrating, and stay up to date on patches and fixes.
Another useful thing is having a good set of tools at your disposal. For example, for tracking user requests, where I work they use wreq, which works out quite well, although I think they had some trouble setting it up (I'm not the sysadmin here, but I talk to them a lot). There are other similar systems, I'm sure several will come up if you search in Freshmeat.net.
And remember that no matter what you do, Systems Administration is a thankless job: when everything is working, you are completely transparent (as it should be!). But when things go south, it's all your fault. But it's a job that is its own reward: there's nothing more satisfying that having everything working perfectly, automated, and well-tuned.