Open Group holds the trademark for UNIX.
They provide the Single UNIX Specification Version 4 [login required; no cost to register].
Books about the Unix environment:
- Learning the Unix Operating System, Fifth Edition by Jerry Peek, Grace Todino-Gonguet, and John Strang (O'Reilly, 2001)
- List of Unix utilities (Wikipedia)
- Unix in a Nutshell, 4th Edition by Arnold Robbins (O'Reilly, 2005)
- The UNIX Programming Environment by Brian W. Kernighan and Rob Pike (Prentice Hall, 1984)
- Unix Power Tools, 3rd Edition by Shelley Powers, Jerry Peek, Tim O'Reilly, and Mike Loukides (O'Reilly, 2002)
- The Linux Command Line [official direct link for pdf download of the book] by William Shotts (No Starch Press, 2012)