Mani or Manichaios was a major religious figure in early Sasanian Mesopotamia (third century CE) renowned as a healer, a visionary, an artist, and a public sage. The community that he founded, the religion of Manichaeism, spread across Eurasia from the late Roman world of the Mediterranean to south China, where it lasted into the early modern period. Due to its prominent success in Central Asia through the latter part of the first millennium, it played a major role as a conduit for ideas, literatures, and practices between east and west. Mani authored many letters in Aramaic during the years of his public mission (ca. 240–270s CE). These letters were collected together to become one 'book' in his new scriptures and the letters were regarded as canonical by followers of Manichaeism. They were translated into numerous languages and read for centuries as guides to life and faith, as well as being utilized in liturgical contexts. No complete version of The Letters survives from antiquity, but there are many remnants and quotations scattered across a diverse set of sources and languages: lengthy citations from individual letters in Latin preserved by Augustine of Hippo and his circle; codex pages preserved in Coptic and Middle Persian recovered during the twentieth century from Egypt and Xinjiang; references throughout other Manichaean literature in Coptic, Sogdian, Uighur, Chinese etc.; a list of titles preserved in Arabic by Ibn al-Nadim; and forgeries used in polemical texts that circulated in the ancient world. This major new study, the first ever dedicated to the topic, contains both a detailed study of the available evidence and a new English translation of all the relevant texts, citations, and allusions. A considerable amount of the material included is either entirely new to scholarship or only known to a small circle of specialists working on original manuscripts in diverse languages.