I like RUTE
. It's a bit dated, but useful.
A good Debian site is http://www.aboutdebian.com/
and since Ubuntu is based on Debian, it should help a bit. It also goes into details about general computing things like networking and whatnot.
For specific "howto do this in Ubuntu" stuff, try the official Ubuntu Guide
Otherwise, you can always search for "linux newbie commands" and you'll get a host of things.