All spring semester standard textbooks for all levels of the education system from grade one all the way up through high school are currently available for download for free. There is a massive number of textbooks in everything from sports for elementary school to high school mathematics. All the books are available in neat PDF files, easy to download. Since these were shared because of the pandemic, only the textbooks from the second half of the semester are available, but this should still be plenty of reading for anyone interested!

If you're interested in learning Chinese from these, you might want to check this article: The benefits of using 语文/國文 textbooks to learn Chinese

For more native textbooks, also check out this article: 866 digital textbooks for expanding your Chinese

Credit to u/mokoyo for highlighting this resource on Reddit.