These resources make open textbooks available online through various digital files that can be freely used because of obtaining permission for copyright or the copyright has expired.
Project Gutenberg is an online library of free eBooks that thousands of volunteers digitize and proofread books. Its mission encourages the creation and distribution of eBooks.
Open Textbook Library has textbooks on extensive college subjects with free licenses with Creative Commons, making it free for anyone to use and change.
UC Davis leads LibreTexts in a higher education institutional cooperative effort to provide an easy-to-use online platform for open educational resources (OER).