Re:Free and Open-Source Mathematics Textbooks
by gmfawcett on Saturday December 21, 2019 @09:19AM
(#21851268)
You might also enjoy /r/mathbooks (and /r/csbooks) on Reddit, where people post links to freely-published math and CS textbooks. Most are not open-source, but all are shared with permission from their authors and publishers.
https://www.reddit.com/r/mathbooks/
https://www.reddit.com/r/csbooks/