Der jüngere Titurel (c. 1270)

Around 1270, Albrecht von Scharfenberg composed the Younger Titurel as a continuation to Wolfram's romance Parzival. Here Albrecht picks up on and extends the notion of Prester John (son of Feirefiz and Repanse de Schoye) as keeper of the Grail, which is held in India. When Parzival visits India to meet his half-brother Feirefiz, he is treated to a description of the land lifted directly from the Letter of Prester John. In other words, Albrecht weaves material from the Letter directly into his romance. 

Read Zarncke’s take on Prester John and "Der jüngere Titurel" (in German) at Google Books (pp. 968-972).