During the early twentieth century, the anonymous author "M" wrote two important spiritual books that influenced a generation of spiritual aspirants, notably Samael Aun Weor:
Some are of the belief that "M" was an American painter named "Thurston," and quote the tradition of Paul Brunton as the source of this identity. Yet, Paul Brunton himself denied it.
"Scattered throughout PB’s notes made from or about this time period are references to an “M;” when queried as to who this was, PB firmly stated that this person was neither Thurston, nor the Rajah, nor Sri Ramana Maharshi, nor anyone else named in his books or notes." - Paul Brunton Philosophical Foundation website
Others have associated "M" with "Master Morya," one of the teachers of H.P. Blavatsky.
Ultimately, the identity of M is not important. What matters is the beauty and power of the knowledge he conveyed in his books.