UC Davis hosts a powerful program for open textbooks (CC-BY-SA-NC). Topics trend towards STEM, but there are some Humanities there as well. Great interface and search with amazing resources.

“A multi-institutional collaborative venture to develop the next generation of open-access texts to improve postsecondary education at all levels of higher learning. The LibreTexts approach is highly collaborative where an Open Access textbook environment is under constant revision by students, faculty, and outside experts to supplant conventional paper-based books. The LibreTexts project is currently directed by UC Davis Professor and project Founder Delmar Larsen.”