Does your public library hold used book sales? Ours has about four a year. Today, the kids and I spent about half an hour there. This is what we bought:
The total cost? Fourteen dollars. For basically the price of one new book, we got all these. A couple are like new, donated to the library by nice people like us. Most are "retired" library books with some wear and tear. But still! Lots and lots of books for the kids and for me, and we supported our library at the same time.
This is how I manage to fill my house with books and still feed my kids nutritious and healthy food :-)