When the price of books increase the stock/supply of books would increase as well due to less people actually buying the books.
For example, say you have 10 books for $5 a piece and all 10 are bought. You'd have no supply of books after. Then you acquire 10 more books and sell them at $15 a piece and only 5 books are bought. Your supply would have gone up because there are still 5 books in stock compared to the previous 0.
