I find it easiest to "learn Linux" through first hand experience. Building NAS units with FreeNAS/Ubuntu, setting up Gentoo, generic trouble shooting, etc. Trust me, Linux will give you plenty of opportunities to "troubleshoot," each time a great learning experience. If all you seek is command line knowledge, use the terminal (Ctrl+Alt+t) in Ubuntu to do all your directory duties (remember sudo su to elevate your privileges to super user). For quickly absorbing a bunch of knowledge, yeah, a self-help dummies book is the way to go, but if you truly want to learn Linux, think: "Sure, I can do it in Windows, but lets get it done in Linux."
And, I've always found this site helpful, if only because Google tells me it is the best:http://www.psychocats.net/ubuntu/index