He is best known for the work Daimonic Reality: A Field Guide to the Otherworld which deals with the paranormal in a similar way that Jacques Vallee, Allen Hynek and John Keel have done in the past.
Harpur's topics deal with Forteana and folklore, daimonic reality, and Western traditions such as alchemy, neoplatonism, hermeticism, and depth psychology.
He claims that not only do myths, poetry, and religions rely on the imagination for new concepts of reality to be created, but so do modern scientific methods and models.
Rosie Jackson, reviewing The Philosophers' Secret Fire for In Trust Magazine describes the book as a "brilliant new study of the imagination",[2] "impassioned, wise, wry, humane, full of dynamic scholarship and inspired argument".
[2] She writes that the work is "[a] fascinating, beautifully written history",[2] "intellectually meaty, overflowing with telling insight and detailed example".