There are books on Ubuntu (such as Ubuntu Kung Fu, and I think there's
Ubuntu For Dummies) available in book shops. I've only seen them in the (for
lack of a better term) BIG Waterstones - in particular the one in Manchester
city center. If your looking for a dead-tree type of thing, I suggest having
a look in your local bookshops for one on the shelf, and having a flick
through to see if they have what you want. I'll take a guess (since I
haven't read them) that these kind of books arn't CLI orientated books
though, as they're aimed towards beginners who need to know GUI stuff too.
Not too sure about websites.
The way I learnt was to start using Terminal for things such as installing
programs with apt-get, or dpkg,and unpacking archives etc., and whenever I
did something I would look for a way to do it in the CLI. I've managed to
pick up a few things doing only that, and it's given me a basic
understanding of how terminal works. You could always try things that way
(or if your system can run a virtual machine of Ubuntu, you may want to do
it in there so as not to break anything accidentally!).