Secret Societies of the Middle Ages: The Assassins, the Templars & the Secret Tribunals of Westphalia
Thomas Keightley (1789-1872) collaborated with T.C. Croker on Fairy Legends.  His other works include Fairy Mythology (1828, still in print), Tales and Popular Fictions:  Their Resemblance and Transmission from Country to Country (1834) and books on Rome, Greece, India, the Crusades, and England.