We know of only a few examples of heart-shaped Books of Hours, all dating from the 15th or the 16th century. Here is the list of the four presented here:
Biblioteca Oliveriana, 1144, 16th c.
KB, Thott 1510, 1550s.
BnF, latin 10536, 15th c.
BnF, Rothschild 297, 15th c.
Rare but attractive - and difficult to produce - type medieval manuscript.