From "Le Livre des Merveilles", 15th century. From the Gallica Digital Library. (Wikimedia)
The legend in its early forms: the arrival in Rome of a patriarch from the east, the chronicles of Otto of Freising, and that famous "letter."
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Sources:
Otto of Freising, Chronicon, ed. G.H. Pertz, MGH SSRG (Hanover: Hahn, 1867), VII, 33, (pp. 334-35), translated by James Brundage, The Crusades: A Documentary History, (Milwaukee, WI: Marquette University Press, 1962). Accessed at Fordham University Internet Medieval Sourcebook.
Prester John: The Legend and its Sources, compiled and translated by Keagan Brewer. Taylor & Francis, 2019.
Baring-Gould, Sabine. Curious Myths of the Middle Ages. Roberts Brothers, 1867.
Heng, Geraldine. The Invention of Race in the European Middle Ages. Cambridge University Press, 2018.