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 a

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/mathbooks/ https://www.reddit.com/r/csbooks/ https://www.reddit.com/r/csbooks/