More than ten years ago, at the National Gallery in London, I purchased a mouse mat depicting a fascinating detail from a Flemish painting (I've never seen the actual painting though).
It is still my mouse mat at work, but only today I discovered there is a whole (regularly shaped) book devoted to this subject:
«The notion of the heart as a "book" containing a person's thoughts, feelings, or memories is one of most prominent forms of heart symbolism in the Middle Ages. In romances, lovers' hearts were inscribed with the name or image of their beloved, while saints' legends celebrated martyrs whose hearts received marks of special divine favor. Clergy were instructed to let their inner scribe copy God's commands onto the pages of their hearts, and ordinary believers prayed for Christ to write the memory of his Passion in their "heart books." Artists portrayed authors holding a heart and a pen, and some late-medieval paintings depicted the sitter as a scribe or reader holding a heart-shaped manuscript codex. Medieval artisans even produced actual heart-shaped manuscript books, some of which still survive.»
the Master of the View of Sainte Gudule: