Zook, Matthew; Graham, Mark (2006). "Wal-Mart Nation: Mapping the Reach of a Retail Colossus". In Brunn, Stanley D. (ed.). Wal-Mart World: The World's Biggest Corporation in the Global Economy. Routledge. pp. 15–25. ISBN0-415-95137-2. [1] Courtesy link to illustration from Matthew Zook's website