
CK-12 is working to establish the development and distribution of quality educational content that can be a core text as well as to provide a flexible context for learning.

FlexBook is an open-content, web-based collaborative e-textbook model. FlexBooks download in the ePub e-book format. Currently there are free FlexBooks available in math, science, technology, and engineering. There are algebra, geometry, and calculus textbooks with teacher support materials. In science, you'll find e-textbooks for biology, chemistry, and physics.
I downloaded the geometry FlexBook. After a quick perusal, it appears to be an adequate base geometry textbook. The images below show how the geometry book renders in several e-book readers on different platforms.
B&N eReader on a MacBook (with highlighting, note, and bookmark)

B&N eReader on a Windows netbook

Adobe Digital Editions on a Windows netbook

Firefox EPUBReader on a Windows netbook

iBooks on iPhone 3GS

You can create and edit your own chapters or even entire FlexBooks.

Watch a CK-12 FlexBooksvideo demo
Register for an interactive webinar "Intro to FlexBooks!"
Friday, July 16th @ 12:00pm PST | Friday, July 23rd @ 10:00am PST
A non-profit organization, the CK-12 Foundation has an objective of reducing the cost of textbook materials for K-12 both in the US and worldwide.
DISCLOSURE OF MATERIAL CONNECTION: http://cmp.ly/0