Also, I'm not trying to get a certificate, I just need to know enough.
Miguel has a good blog post on learning Linux.
http://tirania.org/blog/archive/2011/Sep-06.html
Another great resource is to pick up a Linode VPS and go through the library.
I owe a lot of my start in Linux to the Slicehost articles but Slicehost is going away as it gets eaten by Rackspace.
Skip the courses, they are a waste of time.
http://articles.slicehost.com/sitemap
http://slicehost.com/community/
edit: [clarified]
1. A book (http://www.amazon.com/System-Administration-Handbook-Edition...)
2. An infrastructure (http://openstack.org/)
3. A community (http://serverfault.com/)
Oh, and some fun (http://www.commandlinefu.com/commands/browse)
:)
Some/all of her books: http://www.oreillynet.com/pub/au/678#Books
Her O'Reilly articles: http://www.oreillynet.com/pub/au/678#Books
Any system administrator or general user can learn a lot from Unix Power Tools:
New, 3rd ed: http://www.oreillynet.com/pub/au/678#Books
Cheap, 2nd ed (prolly good enough if $ matters): http://www.abebooks.com/servlet/BookDetailsPL?bi=8354343877&...
Some of this looks good, some looks like crap: http://www.amazon.com/s/&url=search-alias=aps&field-...
It is a commitment, but I recommend reading "The Manual": _UNIX and Linux System Administration Handbook_ by Evi Nemeth, Garth Snyder, Trent R. Hein, Ben Whaley http://search.barnesandnoble.com/Unix-and-Linux-System-Admin...
That'll cover basic Ubuntu sysadmin tasks, which is Debian-based. It's a good place to start.
In terms of more general learning, I suspect that much of it is very domain specific. Are you on a LAMP stack?