Kenneth Robert Henderson Mackenzie was an English linguist and orientalist, as well as a Freemason and co-founder of the Societas Rosicruciana in Anglia (SRiA).
Mackenzie's work is fantastic -- a first-rate, excellent resource for Freemasons interested in the Western Mystery Tradition. First published in 1875, its lustre is undimmed by the years. It is still one of my first go-to sources for information, ahead of Mackey's and certainly ahead of Coil's.
Esoteric, obscure, possibly outdated but very very well researched and very well written (although the 19th century prose does make it feel very outdated).