C.W. Bovill, Caravans of the Old Sahara (London: Oxford University Press, 1935), pp.4-183; Diedrich Westermann, Geschichte Afrikas (Cologne: Greven Verlag, 1952), pp.356-62, 382, 390-91, and passim. A.H. M.Jones and Elizabeth Monroe, A History of Ethiopia (Oxford: Clarendon Press, 1955), pp.44-79; J.C. de Graft-Johnson, African Glory (London: Watt & Co., 1954), pp.68-126; R.Coupland, East Africa and Its Invaders (Oxford: Clarendon Press, 1938), pp.2-43; G.PMurdock, Africa: Its Peoples and Their Culture History (New York: McGraw-Hill, 1959).
