Friends of the Library is the best deal out there. If your in an area that has multiple of these you can stock a private library quick and cheaply if you stop by once or twice a week. Just have to be willing to buy books for later that may interest you, because it won't be there next time you go. Have quite the private library on ancient history and mythology among other subjects.
https://openstax.org/ is what a lot of people are starting to use as free online textbooks. A free US History text is there for anons to peruse.