The repositories listed below contain several thousand open textbooks, covering a variety of disciplines. However, there is some overlap. Because the open licenses permit redistribution, you may find the same resource in multiple repositories.
Faculty Select allows TU faculty to find and access OER content, as well as find and request unlimited-user ebooks for purchase by the library. For OER ebooks, a link is provided to preview the content, along with an option to receive a course-friendly link which instructors can include in their syllabus or Blackboard course site if they choose to adopt the title for their course. For ebooks available for library purchase, faculty can preview the content and submit a short request form asking the library to purchase the book, using their department's allocation of the library collections budget, pending approval by their department's faculty representative.
OER Commons is a "digital public libray and collaboration platform" from the Institute for the Study of Knowledge Management in Education (ISKME). The link above will take you directly to a list of post-secondary OER. NOTE: This site includes materials which cannot be revised or remixed. To exclude these, use the "Conditions of Use" limiter on the left side of the screen. Choose either "No Strings Attached" or "Remix and Share" to only view materials which include all OER permissions.
SUNY's OASIS (Openly Available Sources Integrated Search) allows you to search for many different types of open educational content--textbooks, course materials, simulations, videos, and more. You can also browse OER by subject on the OASIS site.
A program of the California State University System, MERLOT II includes over 80,000 OER, comprising instructional content, learning assessments, and course planning tools. The OER available on MERLOT II span across all academic disciplines.