THE OCCULT CONSPIRACY: the Secret History of Mystics, Templars, Masons and Occult Societies. By Michael Howard
In my researches before starting to write my speculative thriller, THE GRAIL CONSPIRACIES, this is one of the books that I relied on significantly in my early readings on the background of the Knights Templar (or Templars), and other occult or secret societies.
In the British edition which I used (Rider and company, 1989) the title is : THE OCCULT CONSPIRACY: the Secret History of Mystics, Templars, Masons and Occult Societies.
There is also an American edition, also 1989, though with a slightly different title: The Occult Conspiracy: Secret Societies--Their Influence and Power in World History.
Both are by Michael Howard.