This is on top of HN right now, and there is still nobody who has mentioned the fact that the tier-1 book Turtle Geometry <https://direct.mit.edu/books/oa-monograph/4663/Turtle-Geomet...> , is in open access.

Do yourself a favor, and read it today : it is that good. Even though it is quite depressing to see that the revolution in education that the book was predicting in its preface, simply never happened.

I was not expecting to see that the book would culminate in a chapter on general relativity!