Free and Open-Source Mathematics Textbooks
e-Books, to be exact. Some of us strongly prefer to work with paper books. Borrowing those for free from a library is not always an option, so one ends up buying them at whatever price point and the physical state they are available in. Other considerations: it is still often true that you get what you pay for; also, you spend so much time and effort seriously working through a math book that whatever money you paid for it looks like something that does not matter at all.