Dunstan Ramsey spends a large amount of his life chasing after the stories of obscure saints in Europe. For a man who is not even a Catholic, and who has an abhorrence of religion from his childhood experiences of Calvinism, the devotion of a life to this pursuit seems odd. But Dunstan has several particular reasons for his interest in hagiography (the lives of saints).
When Dunstan was in the first World War, he was saved, he believes, by a vision of the Virgin Mary. He was heralded as a hero, because he had performed a heroic act in which he almost certainly should have died. He did lose a leg, and had withered arm his whole life afterwards, but he did survive; and he did not attribute this to luck, but to divine intervention. That this didn't create in Dunstan a profound conversion to a faith is also somewhat odd -- he remains a kind of believer, but a questing, questioning one, rather than a churchgoer.
But before his war experience, Dunstan identified the woman who lived next door to him, Mary Dempster, as a saint. She had been hit on the head with a snow-covered rock meant for Dunstan (then Dunstable). Dunny dodged out of the way of the snowball thrown by Percy-Boyd Staunton, and it hit Mrs. Dempster. Forever after Dunstan bears the guilt of this -- even though, by most people's standards, Percy-Boyd (to become Boy) was far more to blame than Dunstan was. Mrs. Dempster, as a consequence of the hit on the head, gives birth to her baby prematurely, and is never right in the head again. She eventually becomes very mad, indeed, but Dunstan attributes at least three miracles to her (including the raising of Dunstan's brother from the dead). Dunstan feels extreme guilt, especially, for the baby Paul, who grew up with a difficult and inept father and an insane mother. Paul eventually runs away with the circus (the World of Wonders, described in the third book of the series) and is cruelly treated for years. Dunstan doesn't find out the details until later, but he had carried the guilt of Mary's madness and Paul's loss (for he was presumed dead in Deptford) for a great portion of his life.
Since Mary Dempster had been the locus of so much suffering, but had been, to Dunstan, a saint whom he watched with horrified fascination, the choice of a life writing hagiography seems more sound. Dunstan's quest to find out the nature of sainthood -- and, perhaps he hopes, to understand the nature of good and evil, guilt and redemption -- became his life's work. Part of the reason he sticks to it for so long is partially because he knew and consorted with the opposite of Mary Dempster for years -- the one who should have borne the guilt for her madness and the deterioration of her family, the rock-thower Percy-Boyd (Boy). Dunstan spends much of his life in regular contact with Boy, and Boy was the devil (and a charming, rich, handsome one, at that) to Mary Dempster's suffering saint. Perhaps Dunstan felt he needed to learn more about sainthood, because he was regularly contaminated by contact with its opposite.
