A large repository of open educational resources in a variety of disiciplines, level of study, and material type. For textbooks, do a search or browse by subject and filter by "textbook" in material type.
One of the largest open book platform containing over 30,000 books from over 400 publishers. While all books on this platform can be accessed freely, some can be reused.
International collection of texts supported by UNESCO and developed by a team at the US Library of Congress in collaboration with partner institutions in many countries