This book presents an English-language translation of Risālā-yi Muʿīnīya, or the Muʿīnīya Epistle. Risālā-yi Muʿīnīya is one of the earliest known works of Naṣīr al-Dīn Ṭūsī (1201–1274), an intellectual luminary of the 13th century CE. The work is notable for the choice of Ṭūsī’s native Persian as the language of the text. In addition, Ṭūsī 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 Ṭūsī's courtly patrons during his decades-long stay with the Ismaʿīlīs, 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 Ṭūsī was to pen hisown commentary on the work (the Ḥall-i Mushkilāt-i Muʿīnīya, or A solution to the difficulties of the Muʿīnīya) and he used this occasion to discuss his celebrated mathematical formulation “the Ṭūsī Couple” (a concept that he merely hinted at in the Risālā-yi Muʿīnīya).