Twice a year our local library has a book sale. They sell the books that people have donated to the library. They sell them cheap, like 50 cents for a paperback and a dollar for a hardcover. Yesterday, I was able to pick up 18 good books for $10. Most of them will populate my library. Some of them will be given to my children to read (and fill their own libraries).
So, watch for your local library book sale. If you look hard, you will be able to find some good, cheap books.