As an author, Caesarius of Heisterbach is best known as the compiler of a book of hagiography, the "Dialogus miraculorum" (ca. 1219-1223), a collection of 746 miracle stories arranged according to twelve distinctions. The tales are told in the form of dialogues between a monk and a novice. The work was often referred to by preachers seeking material for sermons in the Late Middle Ages. It was extremely popular and was widely distributed, and its popularity was rivaled only, perhaps, by the "Golden Legend" of Jacobus de Voragine. A vision reported in the book provided the source for the iconography of the Virgin of Mercy.