first I would heartily recommend the ubuntu forums. I've been using them since I started with ubuntu (for nearly 4 years ago now), and find all the users helpful, polite, and (to use a french term) 'tres sympa, tres genial' (which kinda translates to helpful and polite, but more so
The community members over the have written a nice guide to the terminal (CLI)
. I would recomend checking it out.
A great book that I picked up is the Linux Desk Reference
, by Scott Hawkins.
I think my copy came with some CD when I acquired a copy of Caldera Linux (back in mid / late 90's.
I still delve into it on occasions even now.
Certification wise, these are not something I have done. I did study and take the exam for certification of XP. But the examination method (multiple choice) seems wrong to me. Especially as there are often more that one method to accomplish the same thing.
I guess with Linux being more CLI oriented it will boil down more to 'do you remember that command and all the switches' as opposed to 'do you remember the 15 different methods to get to the microsoft management console, and it's various bits'.
Add to the fact that some of the tools (ie, user files and settings transfer wizzard) just never seemed to work when I tested them - a dedicated <home> directory on a separate partition, works much better (just tar up the drive, zip it, and move it to your new system, Job done).
If you are really keen to learn how stuff works I would recomend getting a very old version of ubuntu, or a version of debian - as you will probably find that half the stuff doesn't work the way it should and you'll need to spend some time to get it all functioning (admitedly this may seem a little unfair on Debian, as their "freedom" ethic is more stringent than ubuntu's).
This link http://releases.ubuntu.com
will take you to where you can get older versions. In these things like Wifi support are 'fun to get working' in some instances. You will then need to use things like google and your own guile to get them to work.
Good luck 'doing the funky penguin' and welcome to the community.