Iran History 640-1256 (Geographic Name)

Wilber, D.N. Four hundred forty kings of Iran, 1972.

Sykes, P. History of Persia, 1930.

Cambridge history of Iran, 1968.

Britannica Macro.: v. 9, p. 857 ("A second Mongol invasion began when Genghis Khan's grandson Hűlagű Khan crossed the Oxus in 1256 and destroyed the Assassin fortress at Alamūt.")