Hi Will -
Our SS folks have been using TCI's version of an online textbook for a few years. Works well on Chromebooks. Go over to teachtci.com and check out what they have. It doesn't have all the rich features of a true online textbook, but it does deliver info.
Discovery Ed also has a promising product in their social studies tech book http://www.discoveryeducation.com/what-we-offer/techbook-digital-textbooks/middle-school-social-studies/index.cfm