Textbooks from Springer on multiple disciplines.
Springer Initiative: https://www.springernature.com/gp/librarians/news-events/all-news-articles/industry-news-initiatives/free-access-to-textbooks-for-institutions-affected-by-coronaviru/17855960
Springer Press Release: https://group.springernature.com/fr/group/media/press-releases/freely-accessible-textbook-initiative-for-educators-and-students/17858180
Credit for the find: https://hnarayanan.github.io/springer-books/