The heart-shaped book this sitter holds is probably a prayer book; he is depicted before a view of the church of Sainte Gudule in Brussels, where a mass is performed by a priest in the background. The man’s identity is unknown, but he may have been member of a confraternity or guild with a particular devotion to Saint Augustine, whose symbolic attribute was a heart surmounted by a flame.
– Young Man Holding a Book
Master of the View of Sainte Gudule (Netherlandish, active ca. 1485)
The Metropolitan Museum of Art
More examples of heart shaped books here…
[h/t to Book Porn for the lead-in…]