Basically, you are free to copy, distribute, and display the book, as long as you give credit to me. The restrictions are that you cannot use the book for commercial purposes without my permission. You are free to modify and build upon this work, provided that you clearly mark all changes and release the modified work under the same license as this book.
Please visit the Creative Commons website [http://creativecommons.org/licenses/by-nc-sa/2.0/] for the full and exact text of the license, or for an easy-to-understand version. There is even a comic strip explaining the terms of the license.