CK12 has open books for most courses. This is really pretty revolutionary and it is all FREE. Check it out

Originally shared by Esther Wojcicki

CK12 has open books for most courses.  This is really pretty revolutionary and it is all FREE.   Check it out
http://www.ck12.org/blog/new-ways-to-learn-ck12-org-is-integrating-with-google-classroom/