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.
Open Textbook Library - 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.
Open Stax College Textbooks - OpenStax is an initiative of Rice University. A nonprofit organization, OpenStax provides free college textbooks developed and peer-reviewed by educators.
1,000+ Open Textbooks - Duplicates resources found on other sites, such as the Open Textbook Library, but may lead to some additional open textbooks
2017 Open Textbook List (by discipline) - Claybaugh, Z. & Stone, C. (2017). Open Educational Resource 2017 Textbook List. Fairfield, CT: Sacred Heart University.
Open Education: Open Textbook Collections -A collection of hundreds if not thousands of openly licensed textbooks in various formats.
Open SUNY Textbooks - A growing collection of openly licensed textbooks in a wide variety of topics.
College Open Textbooks - Openly licensed textbooks for a variety of subjects. Focus is on first two years of college.
Libre Texts - Open CC licensed textbooks for various disciplines created in partnership by the National Science Foundation and the University of California Davis
OASIS - Openly Available Sources Integrated Search (OASIS) is a search tool that aims to make the discovery of open content easier. OASIS currently searches open content from 73 different sources and contains 173,902 records. OASIS is being developed at SUNY Geneseo's Milne Library in consultation with Alexis Clifton, SUNY OER Services Executive Director.