It used to be customary for school children to hire out school books for free directly from the school. Nowadays, however, parents must buy most of the books themselves. Tip: Put a note on the school's board explaining that you are willing to buy books 2nd hand. There are a lot of older kids who would be glad to get rid of their old books.
Some schools have their own library (Schulbuecherei), from which children can hire out books for free. Most libraries carry books related to school subjects and children's books. Encourage your kid to use these facilities; libraries are a good place for doing homework or reading in quiet surroundings, too.