The following sources treat the historical underpinnings of the rumor recorded by Otto:
Henry H. Howorth, “The Northern Frontagers of China: The Kirais and Prester John.” Journal of the Royal Asiatic Society, vol. XXI (1889): 361-431.
Douglas Morgan Dunlop, “The Karaits of Eastern Asia.” Bulletin of the School of Oriental and African Studies 11 (1944): 276-289.
Denis Sinor, “Western Information on the Kitans and Some Related Questions.” Journal
of the American Oriental Society 115.2 (1995): 262-269.
Gustav Oppert, “On the Kitai and Kara-Kitai.” The Journal of the Ethnological Society of
London 2.2 (1870): 97-106.