Good history on secret societies during the Qing dynasty and their role as precursors to the twentieth century revolutionary movements. It was probably the most thorough treatment of this subject until Elizabeth Perry's work. One of its biggest strengths is that the organizational structures, member profiles, and rituals are described in exceptional detail. Lots of attention is also given to the relationship between secret societies, other voluntary associations, and dynastic law, so the reader gets a good sense of how secret societies fit within the broader social contexts. But it's probably too descriptive to work as useful social science, and the liberal references to Simmel gives the impression that Davis wanted to write a work of sociology as much as a work of history. (I also think the title is a reference to Hutton Webster's classic anthropology book "Primitive Secret Societies"). Most of the discussion on religion, especially the millenarian strands, and secret societies is pretty superficial, but that's probably not a huge drawback considering that later China scholars have written a lot about this area (e.g. David Ownby).
tl;dr How did you even come across some dusty old thesis-turned-book on Chinese secret societies? You probably won't be terribly disappointed if you're already searching for this crazy arcane stuff.