The Mahatma Letters to A. P. Sinnett is a foundational work of the Theosophical Movement. First published in 1923 by A. Trevor Barker, this volume gathers most of the letters sent by Mahatmas Morya and Koot Hoomi to A. P. Sinnett and A. O. Hume between the years 1880 and 1884. The original letters are in the British Library.