The price is still right: 15 sites for free digital textbooks — from campustechnology.com by Dian Schaffhauser
We’ve collected an updated roster of resources that offer quality learning content without the high price tag of traditional texts.

Excerpt:

The demand for free learning content may be loud and clear now, but, back in 2013 when Campus Technology first surveyed the top sources for free digital textbooks, the OER world seemed a quieter, less tweeted place. What hasn’t changed, though, is that faculty and students still want to know where to go to find the goods.

The following list offers 15 sources of quality digital content to use in your courses without worrying about the price tag.