
It is not fashionable nowadays to speak of the merits of the command line, in an age where things like streaming video and Aqua are an integral part of our daily life. However, I do not think that typed-in commands must necessarily be consigned to the dustbin of computer history. Of course, I am not suggesting that we all drop X and Windows and pretend like we are living in the early eighties. The command line interface still has much to offer us, and many of its benefits simply cannot physically be emulated or even replaced by graphical ones.
If you really want to learn about the power of the Unix command line, there's no better way than browsing through "Unix Power Tools" from O'Reilly. It is a unique book, after the introductory chapter the rest is arranged as semi-hypertext so you can browse to whatever tips you are interested in easily. It also has some information in it posted in Usenet when it was still a use-ful place.
The latest 3rd edition is rather expensive at about $70, but used editions of earlier versions should be affordable from various online bookstores or eBay. (I own the 2nd ed, but have heard the 1st edition is the best because it includes a special chapter on awk that was removed in favor of a Perl-coverage stub.)