There was no "Prester John" (Presbyter Ioannes, "Priest John"). That didn't stop him from writing a letter that circulated throughout Europe in the middle ages. You have three versions to read.
The first version is Michael Uebel's translation of the first Latin letter of Prester John (from Uebel's book):
The next file contains two texts: first, a translation of one of the early Latin versions of the letter, and then a translation of a later and more elaborate French version. You should read both, but concentrate on the Latin version.
The original letter seems to have been written between 1165 and 1170; there are two references to Prester John that might predate (and have inspired) the letter; for details on the long and complicated history of the legend and texts, see:
Brewer, K. 2015 Prester John: The Legend and its Sources. Surrey: Ashgate.