Martin Pasko, The DC Vault: A Museum-in-a-Book with Rare Collectibles from the DC Universe, Philadelphia, Pennsylvania, Running Press, 2008, p. 46, ISBN 0-7624-3257-8.
«During [World War Two], overall circulation tripled, as servicemen added comics to their reading habits. At the height of the war, many titles were selling over a million copies a month. Superman topped the list, of course—at first.»
