Nearly 1.5 million college students to use free textbooks this school year

"Nearly 1.5 million U.S. college students are expected to save an estimated $145 million in the 2017-18 academic year by using free textbooks from OpenStax, the Rice University-based publisher of open education resource materials. …

… By this fall more than 11,000 faculty members at 4,200 universities or colleges will have adopted OpenStax textbooks for 8,500 courses." …

https://openstax.org

Looks like books for a selection of frosh-level courses and some remedial math courses:
https://openstax.org/subjects