There are two ways to create a book bundle or box for sale on your site as long as you have a PRO subscription.
First, you can create a new book listing in the Books section to describe the multiple books that you would like to sell as a set. You will need to upload a picture of the bundle and set a specific subscription and pricing in the Sell Direct tab for the book.
Secondly, you can sell a book bundle as "Merch" if it includes special packaging or other items. You can set up this item for sale in the Merchandise section.