Piers Paul Read's 'The Templars' is a good orthodox account of the order's history which examines the emergence of the Templar Myth, but is really dedicated to rejecting it. Peter Partner's 'The Murdered Magicians' is a fuller treatment of the myth, which he explodes.
Umberto Eco deals with the story in 'Foucault's Pendulum' as well; one of the principles Belbo, Casaubon and Diotellevi work upon in framing their own conspiracy theory is that 'The Templars are involved in everything'.
