sysadmin info: O'Reilly Safari
O'Reilly has most of their books (plus some from other publishers) available online at safari.oreilly.com. This is a subscription service; you pay by the month or annually with a discount. BUT there is a two-week free trial - during which time you should be able to get your system up and running.
The catch is that you have 10 available slots for books, and you can't remove a book for 30 days. So you can see at most 10 books during the trial (some books take more than one slot). I think that's still pretty good for free!
I have one non-recommendation: Essential System Administration tries to cover all *nix operating systems and so wasn't able to go into depth on any single topic as much as I would have liked. On the other hand, I was happy with Postfix, in which I found all the configuration answers I was looking for.
I'm actually a paid subscriber at the 5 slot level. It's a bit pricey and I definitely prefer paper books to e-books, but it's great if you just need something for a short while, if you want to preview a book before you buy it, or if the hyperlinked contents and index make it a more convenient reference.
Roy