Tucker, Spencer; Roberts, Priscilla Mary () [2005], Daniel Ramos, ed., World War I: Encyclopedia(Digitized by Google Books online), United States: ABC-CLIO, p. 312, ISBN9781851094202
Dirk Steffen, "The Holtzendorff Memorandum of 22 December 1916 and Germany's Declaration of Unrestricted U-boat Warfare." Journal of Military History 68.1 (2004): 215–224. excerpt