A nonprofit group at Rice University and one of the more prominent OER sites, OpenStax provides textbooks for AP and college classes. Funding is provided by various foundations and through "partnerships with for-profit companies that provide sophisticated, optional online homework at a low cost". All textbooks are free and peer-reviewed; optional online homework is not free.
In response to the availability of used textbooks and the growth of open educational resources publishers have shifted towards selling access codes. Access codes open web platforms which contain online quizzes, videos, homework assignments, and more, yet cannot be reused by another student. This study examines "the potential impact of access codes" and analyzes the student view of the shift from textbooks to access codes.