The Mahatma Letters to A.P. Sinnett is a book published in 1923 by A. Trevor Barker. The work consists of one-hundred and forty-five letters written by masters Koot Hoomi and Morya to Theosophical author Alfred Percy Sinnett (author of the much renowned work Esoteric Buddhism) between the years of 1880 and 1884. The letters are particularly important to the Theosophical Society and their wider movement because of their long elaborate discussions of the theosophical cosmic order and spiritual hierarchy. It is generally regarded as the most authoritative work of theosophy that is accessible to the general public.