Check out the "Community" section of any of the major VPS hosting companies - some of them have really detailed admin tutorials for their customers.

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 .