A must read! A shameless plug for my father's new book - Gregorius: An Incestuous Saint in Medieval Europe and Beyond.
The striking (and shocking) story of Gregorius, a child born of incest who later marries his mother, undertakes a massive penance, and eventually becomes both pope and saint, is a European theme. The apocryphal story probably came from medieval England or France, but is found in most European languages in all kinds of forms, from major literary works down to short folktales. In modern times the best known (but not the only) version is that by Thomas Mann, The Holy Sinner. It is emphatically not a variation on the Oedipus-legend; instead it uses the strange and paradoxical tale of the good sinner to demonstrate the effects of original sin, and to show how the human propensity to sin in general can be coped with, just as long as the sinner does not despair. This book traces the story in its variations from Iceland to Egypt and from the twelfth to the twenty-first century.
More details on the OUP website. It is very good indeed.