While both of the above comments are accurate -- and Zamboni's is the general answer -- Wittgenstein's work is the most widely referred to as "the Tractatus" amongst academics in not just philosophy but a variety of different disciplines. When I heard Reid's comment it struck me as odd becuase, as the first author notes, the Tractatus isn't specifically about religion. Nevertheless I was confident that it was Wittgenstein's book that Reid was looking at.

It gave me a happy academic-geek feeling.