Written in the 19th century, Addison gives copious eye-witness descriptions of events (both Christian and Moslem) without comment. Details of sources are fully listed for each chapter.
So we see the zeal of the early Templars, whose task was to protect Christian pilgrims when visiting Jerusalem. Later, the Templars gained immense wealth and influence, although individual Templars took a vow of poverty. This wealth came from gifts from donors who felt they were supporting a worthwhile cause (and gained some spin-off). But then there are accounts of Templars' heresy and bizarre rituals. Were these claims of heresy needed to weaken the Templars' reputation in order to get at their wealth? Heresy does not square with the early zeal, apparently based on Christian beliefs. Many detailed accounts are given, so make your own mind up. I was surprised at the high level of gynophobia in many of the accounts.
I was less interested the second part of the book which focuses on the Temple Church in 19th century London. Addison does not say that the original Knights Templars were disbanded in the 14th century.
Having read this book, do I now understand the Knights Templars? No, not fully. The Knights Templars evolved and changed as an organisation - which probably accounts for some of the contradictions in their customs. Undoubtedly there was some corruption. They evidently had a medieval mindset which eventually sat uneasily in a world of growing trade and science.
Perhaps there are more recent accounts that look afresh the Knights Templars story? I hope so. Addison is strong on facts but weaker in interpretation.