Flexbook is a tool that CK-12 is using to allow users the ablity to develop their own online textbooks.The Flexbook system allows an individual to author their own textbook taking pieces from other books in the system, or complelely from scratch.
As an example, the state of Virginia has released their first open-source textbook for public review. You can get the specifics of their first attempt here:
Governor Schwarzenegger initiative to develop online textbooks for high schools in California has focused the light on this important topic and Flexbook might be something you will hear more of in the future.
CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide.
You can get more information about Flexbook from their web site and you can watch a short video here: