He is considered to have written the two most comprehensive books on the subject, The Trial of the Templars (1978) and The New Knighthood: A History of the Order of the Temple (1994).
Ia menulis dua buku yang dianggap paling lengkap mengenai subjeknya tersebut, The Trial of the Templars (1978) dan The New Knighthood: A History of the Order of the Temple (1994).