This book presents an English-language translation of RisÄlÄ-yi MuÊ-Ä«nÄ«ya, or the MuÊ-Ä«nÄ«ya Epistle. RisÄlÄ-yi MuÊ-Ä«nÄ«ya is one of the earliest known works of Naṣīr al-DÄ«n ṬūsÄ« (1201-1274), an intellectual luminary of the 13th century CE. The work is notable for the choice of TÌ£uÌsÄ«'s native Persian as the language of the text. In addition, TÌ£uÌsÄ« organized his volume into a four-part structure, which went on to become a popular template for the Islamic astronomers who succeeded him.   This book helped ensure the patronage of TÌ£uÌsÄ«'s courtly patrons during his decades-long stay with the IsmaÊ-Ä«lÄ«s, as well as the continuation of his remarkable career under the first Ilkhanid rulers of Persia. This translation helps make this notable treatise accessible to English language readers. It is among a handful of English translations of major astronomical works dealing with hay'a/cosmography in the Islamic world.  Subsequently TÌ£uÌsÄ« was to pen hisown commentary on the work (the Ḥall-i MushkilÄt-i MuÊ-Ä«nÄ«ya, or A solution to the difficulties of the MuÊ-Ä«nÄ«ya) and he used this occasion to discuss his celebrated mathematical formulation "the TÌ£uÌsÄ« Couple" (a concept that he merely hinted at in the RisÄlÄ-yi MuÊ-Ä«nÄ«ya).