"Box bookstores" refers to large retail stores that primarily sell books, often located in warehouse-like buildings. They are typically known for their wide selection of books and discounted prices. Full definition
I've run into this with other big box bookstores around the country. (madgeniusclub.com)
Note: Don't get discouraged if big box bookstores turn you away; many are restricted by corporate policies from helping authors outside of publisher - arranged collaborations. (selfpublishingrelief.com)