OER are freely and publicly available teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. For example, instructors may download the material, tailor it to one’s course, save a copy locally to share with one’s students and share it back out with attribution. OER can include textbooks, course materials and full courses, modules, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge. An open source (or open) textbook is a textbook which is OER.
Search these collections for textbooks that match your course objectives. Some sites are more user-friendly than others! The sciences are generally better-represented than the humanities.