I would recommend looking into the content picker field. You can attach this to your Box type and then each time you create a box you can select all the books you want to add to your box. You will need to have created all the relevant books before creating your box. You can have a setting on the content picker field so that it only displays certain content types, I suppose in your case it would be the Book content type
David Hayden has a nice introduction to the content picker field, available here: http://www.davidhayden.me/blog/content-picker-has-content-type-filtering-in-orchard-cms