See:

Press Release: CK-12, Leading Non-Profit Provider of Digital Textbooks to Schools, Makes the Grade with California’s Free Digital Textbook Initiative — eSchoolNews.com

CK-12 FlexBooks:

CK-12's flexbooks

CK Foundation

ck12.org

Key Benefits

Access to free textbooks
High quality educational content created by educators
Content customized to reflect “today” and the different needs of students
Quality ensured by CK-12’s Community of Educational Practitioners
Increased pedagogic choice for all teachers, aligned to state standards as well as developmentally correct content
Supported by publishing tools that facilitate quick and easy content creation and distribution
Collaborative learning via a community where authors, teachers, and students create, access, share, rate, recommend, and publish