Linux is a freely distributable version of Unix.
(Unix was originally developed as a multitasking system for minicomputers and mainframes in the mid-1970s, at the same time as the C programming language.)
It was started by Linus Torvalds, Helsinki, in 1991.
The book (Bible!):
by Matt Welsh a. o., editor O'Reilly, ISBN 1 56592 469 x.