You can learn more about the history and evolution of UNIX by consulting these books:
Dunphy, Ed. The UNIX Industry and Open Systems in Transition. 2nd ed. New York: Wiley, 1994.
Libes, Don, and Sandy Ressler. Life with UNIX. Englewood Cliffs, NJ: Prentice Hall, 1989.
Lucent Technologies. “The Creation of the UNIX Operating System.” http://www.bell-labs.com/history/unix/
Ritchie, D.M. “The Evolution of the UNIX Time-Sharing System.” AT&T Bell Laboratories Technical Journal, vol. 63, no. 8, part 2, October 1984.
Salus, Peter. A Quarter Century of UNIX. Reading, Massachusetts, 1994.
The UNIX System Oral History Project. Edited and transcribed by Michael S. Mahoney. AT&T Bell Laboratories.