The Linux Command Line (2nd Edition) by William Shotts.
From the webpage:
Designed for the new command line user, this 537-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.
Free PDF as well as print version from No Starch Press.
Downloads issues tonight but from my memory of the first edition, this is a must-download volume.
I first saw this in Christophe Lalanne’s A bag of tweets / July 2013.