When I was a kid I'd go to garage sales with my mom and there more often than not was a box for books that were either .10 or .25 each. There is a used book store here in town that has some good prices though I haven't bought any for a while. Space isn't expandable.
I buy mostly ebooks now and if they are 100% off or outright free I even save $ which is good when you're on a fixed income.