The following references will provide a very nice overview of the field – from general theory to in-depth case studies:
1) “Six Sigma: The Breakthrough Management Strategy Revolutionizing the World’s Top Corporations” by Mikel J. Harry, Ph.D. and Richard Schroeder, Doubleday 2000.
2) “Jack Welch and the GE Way: Management Insights and Leadership Secrets of the Legendary CEO” by Robert O. Slater, McGraw-Hill 1998.
3) “Six Sigma for Everyone” by George Eckes, John Wiley and Sons 2003.
4) “Six SIGMA Leadership Handbook” by Rath and Strong Staff, John Wiley & Sons 2003.
5) “Quality Control Handbook” by J. M. Juran, F. M. Gryna, and R. S. Bingham. McGraw-Hill Book Co. 1979.
Although this set of references is not fully comprehensive, it will certainly get you started in the right direction. Best of luck in your Six Sigma journey.