What I love about Ubuntu is that there is SOOOOO much support right out of the box.
And this happens to be one of my biggest pet peeves for someone looking to learn especially in this arena (security). The issue with "support" when learning in this environment is that the learner will learn to skip truly learning as support is readily available. That type of reliance is a moot point when faced with making mission critical and time sensitive decisions in the real world.
I think someone learning Linux should learn as much as they can by trial and error. You WILL NOT HAVE books in the field when performing penetration testing in say a client's environment. So the best bet is to aim to learn it as best as one can understand it on their own accord. Avoid using package managers (yum, YaST, apt-get, pkg_add) and configure things from scratch. Get used to the commands, get used to trouble shooting based on what you see. Imagine NOT being able to connect to a support forum. What then?
Linux, BSD, Solaris, etc., are not difficult at all and too many people have used forums as a form of crutch. We all use forums but its better to learn it from the ground up. Make scenarios for yourself and understand what you are doing. E.g. week one create your own webserver. Do this without package managers. Week 2, make it a virtual hosting server (multiple domains), following week, add email, next add say monitoring. And so on and so on.
Get used to doing things on your own by setting up tasks. you end up familiarizing yourself with tips and tricks not often found in books and your reliance on forums and or books will diminish as experience grows