OpenStax
OpenStax is an initiative of Rice University that publishes peer-reviewed, openly licensed textbooks for common college subjects, from introductory biology and physics to economics, psychology, and statistics. The books are free to read online, downloadable as PDFs, and available in low-cost print editions.
Most titles are released under a Creative Commons Attribution (CC BY) licence, which lets you reuse and adapt the text for any purpose, including commercial work, as long as you credit OpenStax. A few titles carry a NonCommercial ShareAlike licence instead, so check the licence on each book before you build on it.
The structured chapter-and-section format makes the material straightforward to split into passages for retrieval, which suits tutoring and study-support RAG systems.
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.