The Linux Command Line by William Shotts – Fifth Internet Edition Available Now!
Designed for the new command line user, this 555-page volume covers the same material as LinuxCommand.org but in much greater detail. In addition to the basics of command line use and shell scripting, The Linux Command Line includes chapters on many common programs used on the command line, as well as more advanced topics.
Released under a Creative Commons license, this book is available for free download in PDF format. Download it here.