This is a "classic" from the field of Conspiratorial studies, which I've started many times over the last twelve or so years but never finished until now. Written by author Idries Shah under a pen name ("Arkon Daraul"), this is an old book with some interesting bits to it. Shah plays a little fast and loose with many of his allegations, but he has something of value to add. He claims to have had contact with several of the societies he writes about, but is vague with details. He also fails to cite many references, and there's no bibliography, which would have made this a five-star book for me -- if he's wrong, at least with his sources handy you can check for yourself.
Shah's main thesis seems to be that most secret societies contain at their core some ecstatic ritualized experience which is what transforms the member into someone new. That's basically the definition of an esoteric society, so it's nothing new, but it's interesting how many societies he claims this for. He also lists the Sufi (Islamic cabbalists) and Tibetan Buddhism as secret societies, which is actually pretty funny. An entertaining read, if a little difficult to slog through sometimes. Definitely recommended as a starting point, as long as your willing to check your facts.
It covers the Assassins, the Knights Templar, and the Rosicrucians in quite some detail; the Sufi, the Mithraists, the Skoptsi, the Carbonari, the Garduna, ancient Mystery cults, the Decided, the Yezidi, the Tibetan Buddhists, the Thuggee, the medieval witches, and the Vehm are given shorter investigations.