Prester John comfortably enthroned in East Africa - Detail from the Queen Mary Atlas, MS 5415 A British Library
On the many fantastic additions to the Letter of Prester John (Dragons! Strange Bakeries! Etc!), and on the theories around it.
Sources:
Prester John: The Legend and its Sources, compiled and translated by Keagan Brewer. Taylor & Francis, 2019.
Nowell, Charles E. "The Historical Prester John." Speculum 28, no. 3 (1953).
Romm, James S. The Edges of the Earth in Ancient Thought Geography, Exploration, and Fiction. Princeton University Press, 2019.
Wang, I-Chun. "Alexander the Great, Prester John, Strabo of Amasia, and Wonders of the East." CLCWeb: Comparative Literature and Culture 14.5 (2012).