As a new command-line user, I find Mark Sobell’s A Practical Guide to Linux Commands, Editors, and Shell Programming an indispensable resource next to Google. I picked up a handful of Linux books, but this is the only comprehensive text that focused mainly on command line.
After reading the book from cover to cover, I have learned the power of using command line and how not daunting the black screen with white text appears to be. Part V, “Command Reference,” makes this book a must-have guide next to my office desk, especially with the cover of Mac OS X. Highly recommended.