Open Textbook Library
The Open Textbook Library is a searchable catalogue of free, openly licensed textbooks, developed by the University of Minnesota's College of Education and Human Development. Open textbooks are funded, published, and licensed to be freely used, adapted, and distributed.
Titles span common college subjects, and each entry links to the full text along with peer reviews written by faculty. Licences vary by book, and most are Creative Commons, so confirm the terms for any title you plan to reuse.
Related sources
LibreTexts
One of the largest open textbook platforms, covering higher education with some K-12 material. Hosts interlinked textbooks and reference material across science, mathematics, humanities, and social sciences. Mostly NonCommercial, so commercial use is ruled out.
Mason OER Metafinder
A discovery tool that runs a real-time simultaneous search across 21 different sources of open educational materials, rather than searching a static database. Useful for finding content to ingest, not a dataset in itself.
MERLOT
A curated collection of online learning and support materials, plus content-creation tools, led by an international community of educators, learners, and researchers. Points to resources hosted elsewhere across many disciplines.
MIT OpenCourseWare
Teaching materials from more than 2,500 MIT undergraduate and graduate courses, published freely online. Includes lecture notes, assignments, and exams. Released under a NonCommercial licence, so it is free to use for non-commercial purposes only.