This is one of several editions of John Michael Greer's popular “The Element Encyclopedia of Secret Societies”. It's a highly informative but above all entertaining study of ancient mysteries, lost civilizations and hidden history.
The emphasis is on Western secret societies, not all of them occult. Political and fraternal societies have also been included, plus a few that defy easy classification. In fact, the number of groups covered is downright staggering, and include many I never heard about before: the Knights of the Flaming Circle (an U.S. anti-Klan group which excluded White native-born Protestants from membership), the Order of Camels (a U.S. fraternity which smuggled alcohol during Prohibition), the Ribbonmen (violent Irish avengers against exploitative landlords and factory-owners) or the Order of the White Rose (yes, they were Jacobites). If you find secret societies boring, don't worry, there are also entries on all the usual goodies: Lemuria, Atlantis, UFOs, ley lines, the Shakespeare authorship controversy and euhemerism. But yes, secret societies *do* dominate the narrative!
Greer is refreshingly skeptical to most of the phenomena he covers, and often criticizes conspiracy theory. Don't expect any “sensational” information on Freemasons, Skull & Bones or the Illuminati. Well, maybe some. It turns out that many people are initiated into 33-degree Scottish Masonry mostly to become eligible for membership of the Shriners, a Masonic sub-branch notorious for its heavy drinking bouts! No prohibition there.
I got the impression that Greer has included some pretty outlandish “rejected knowledge” claims for the sheer fun of it. What are we to make of Rainbow City, an ancient Martian city, made of multicolored plastic blocks, that lies hidden somewhere beneath the icy wastes of Antarctica, at least according to the Hefferlin Manuscript? Sometimes, Greer is perhaps to speedy and dogmatic in his denunciations, as when he writes about Traditionalism without mentioning Frithjof Schuon and other important thinkers of that milieu. It's also interesting to note that the author is, ahem, a 33 degree Freemason himself…
That being said, I nevertheless recommend this encyclopedia. It's really useful as a serious reference work. Swedish readers might see it as a good complement to “our” classical encyclopedia of matters occult and spooky, Poul Fersling's “Naturligt Övernaturligt”.