What I start with whenever I need to do Ubuntu admin is the Linode Library ( http://library.linode.com/ ). Their guides are fairly comprehensive for most sysadmin tasks, easy to follow and kept up to date, eg http://library.linode.com/lamp-guides/ubuntu-11.10-oneiric .
Everyone's environment is radically different. At my last job we had LDAP & Active Directory. At my current job we have Active Directory + NIS. There are a few constants like DNS, DHCP that you should probably learn, but beyond that you've got chef, cfengine, puppet and spacewalk, and that's just for package management. There's so many different permutations of setting the environment up that you can't learn it unless you learn as you go.
Some constants:
- vi - always installed
- (ba)sh - It's the default shell in most places, though I have seen csh
- perl - also always installed
- DNS - Most groups with a sysadmin have DNS rather than NetBIOS type resolution of names
- SMB - Because you have your Windows users
- NFS - Linux file sharing
- DHCP - Some people use AD for DHCP and then just static the Linux boxes as it's easier, and you get autoregistration in DNS. For folks with more Linux boxes you might have a Linux DHCP server.
- AD - You'd be hard pressed to find a pure *Nix shop
- LDAP - Depending on the situation, AD could be primary or these could be 2 standalone authentication systems with matching accounts.
Hanging out with other sysadmins is better than a book. I find that information in books becomes outdated pretty quickly. Try new stuff in a development environment, and then pray they work the same when you go to production (often they won't - so follow the golden rule - back up everything).
As for specifics there is a lot of good stuff in here: http://www.debian.org/doc/manuals/debian-reference/
If you come from windows avoid using X and GUIs as that really shouldn't be needed in a server environment.
2nd edition, copyright 2010, covers 10.04 LTS; the 1st edition copyright 2009 covers 8.04 LTS.
