We know of only a few examples of heart-shaped Books of Hours, all dating from the 15th or the 16th century.Additional photos of several other such books are posted at damien kempf, via Erik Kwakkel. For a general introduction to Books of Hours, check out this 12-minute video:
This video "presents a brief documentary about the patronage, content and use of medieval Books of Hours. The Biekorf ("Beehive") Library, best known for its collection of cistercian manuscripts from Ten Duinen en Ter Doest, holds 21 medieval Books of Hours."Video via Erik Kwakkel.