What's the use of ancient books in the legend of Shushan?

It's no use. It should be used to brush silver coins. It was 10 silver when it was bought and 1000 silver when it was sold, but it is unlikely to appear. Booksellers can spend 50 silver coins on their brushes, and you will make money when you brush them. You can brush 1000 silver coins for 20 times, but you will lose money if you brush 19 times continuously, because it will cost 60 more.