Publishes a collection of peer-reviewed, freely available online textbooks. It is an initiative of Rice University. They have a limited number of disciplines covered, but there is a gold mine of supplemental resources linked under each.
OpenStax
OpenStax Support – Resource page to get started in using the platform or find help from the question bank.
Free Textbooks From Rice OpenStax: Too Good to Be True? – A discussion of Open Stax including its origins and the groups involved.
Keywords: Open Educational Resources (OER) & Open Access