There are many, many good references available to give you a deeper look into Six Sigma. Here are few that I have found to be useful:
The Six Sigma Handbook, Revised and Expanded by Thomas Pyzdek (McGraw-Hill)
This is a very complete and comprehensive look at Six Sigma. It covers a good many of the statistical tools you can use in a Six Sigma program, providing a pretty solid picture of Design of Experiments. It also covers the types of management systems that can complement a Six Sigma program.
The Six Sigma Way by Peter S. Pande et al. (McGraw-Hill)
Here is another thorough look at Six Sigma. The Six Sigma Way describes how you can adapt and implement Six Sigma across a variety of conditions and environments. The emphasis here is on designing the program to align with specific business goals and objectives. A large part of the book is devoted to implementing Six Sigma in this way, and the authors don't shy away from the complexities you'll probably face when using Six Sigma to shape corporate strategies and directions.
Six Sigma for Everyone by George Eckes (Wiley)
This work provides more of a general overview to the theory and practice of Six Sigma. While this book is short and keeps to a general tone, it does provide a good look at the technical and design aspects of the program, featuring lots of statistical charts, graphs, and tables often used in Six Sigma analyses.
Statistics for Six Sigma Made Easy by Warren Brussee (McGraw-Hill)
The realm of statistics that can be applied to Six Sigma design and analysis is wide and deep, and this book addresses many, many statistical tools and techniques. I don't know if you can call the understanding an easy oneunless statistics is your fortebut this book explains things in a clear and concise way and does a good job of explaining how certain approaches are more appropriate than others for specific types of Six Sigma projects.
What Is Lean Six Sigma? by Michael L. George et al. (McGraw-Hill)
This is a little bookjust over 90 pagesbut anyone brand-new to Six Sigma will find that it provides a handy overview. It's cleanly written and illustrated and takes the reader through the main topics that surround Six Sigma.
Six Sigma for Green Belts and Champions: Foundations, DMAIC, Tools, Cases, and Certification by Howard S. Gitlow and David M. Levine (Prentice-Hall)
From the title, one would think that this is a general-overview type book. But at over 700 pages, it's really a very comprehensive and detailed look at Six Sigma. Its focus is on information useful to Champions and Green Belts, and from that angle, the authors take a solid look at what it takes to sponsor a Six Sigma program, what it takes to run one, and what kinds of activities and considerations are needed to provide ongoing support for the program.