Michael Luck & Mike Joy 
Introducing UNIX and Linux [PDF ebook] 

An introductory, tutorial style text covering the basics of UNIX and Linux for the complete beginner, this is a comprehensive and well written introduction to these operating systems. It assumes no prior knowledge of programming nor any experience of using computers. UNIX and Linux are two of the most commonly used operating systems within the educational and corporate worlds and are growing in popularity. This book covers all the basic constructs and commands of UNIX and follows the 1993 POSIX.2 International Standard.