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) Six Sigma Leadership Handbook by Rath and Strong Staff, John Wiley & Sons 2003.
3) Jack Welch and the GE Way: Management Insights and Leadership Secrets of the Legendary CEO by Robert O. Slater, McGraw-Hill 1998.
Although not casual reading, Juran’s Quality Handbook provides a very nice overview of the various statistical tools, methods, and procedures commonly associated with the practice of Six Sigma.