Jonathan Riley-Smith, The First Crusade and the Idea of Crusading (London, 1986), pp. 17–22

Joseph R. Strager, ‘Feudalism in Western Europe’, in Rushton Coulborn, ed., Feudalism in History (Hamden, Conn., 1965), p. 21; Michael Gaddis, There is No Crime for Those Who Have Christ: Religious Violence in the Christian Roman Empire (Berkeley, Los Angeles and London, 2005), pp. 334–35; John Keegan, A History of Warfare (London, 1993), pp. 283, 289