A very informative and easy to read book, which should be applicable for anyone interested in any esoteric work; not specific with any particular order or path, instead focuses upon the tasks of the iniate and the roles of the group and society.
It examines how and why esoteric schools have restricted admission to their secret societies and orders, and shrouded their practices in mystery, revealing every aspect of these secret organizations and the training they offer to initiates. Written long ago, all of the info is still clearly understandable and most assuredly applicable as it was  back then.