This list is a starting point. These are a few places where you can search for openly licensed textbooks. Some of these repositories contain only textbooks, while others contain other learning materials too.
"Freely available open education resources (OER) that supplant conventional paper-based text books. LibreTexts currently encompass twelve widely used college-level disciplines from chemistry to humanities."
A free and open resource designed primarily for faculty and students of higher education. Links to online learning materials are collected here along with assignments and comments to enhance the teaching experience of an exercise. From the California State University system.
This digital hub created by the Institute for the Study of Knowledge Management in Educationoffers 73,000 kinds of OER, along with tools for creating OER, training on how to use OER and the ability to create OER "Commons" — groups that share resources for a given purpose.
OpenStax is an initiative of Rice University. A nonprofit organization, OpenStax provides free college textbooks developed and peer-reviewed by educators.
Note: Faculty and students may continue to view and download their content from cnx.org until the end of 2021 and in 2022, All CNX content will be available on Internet Archive.
A catalog of open access textbooks to be reviewed by faculty members, made available by the University of Minnesota's College of Education and Human Development. All textbooks in this catalog are openly licenses, complete, and available in print at very little cost.
The US Department of Labor's Trade Adjustment Assistance Community College and Career Training (TAACCCT) program are providing this site containing free and open learning materials and program support materials for job-driven workforce development.