A library has a certain number of books. The books can be arranged on shelves with 9 books per shelf on the first day. They find that they can rearrange the books into a different number of equal shelves each day for 7 days in total (including 1 book per shelf and all books on one shelf). On the 8th day, they cannot find a new way to form equal shelves. What is the smallest possible number of books?