The Hairy Heart: A Guide to Wizards Who Won't Commit is a Wizarding self-help book. It is assumed that the book's purpose is to assist a witch or wizard to avoid having "a hairy heart," which is a Wizarding expression meaning to describe someone who is cold or unfeeling or who won't commit to a relationship.
The origin of the expression "to have a hairy heart" roots to the fable The Warlock's Hairy Heart, where the character, the Warlock, had hid his heart in his dungeon. This led him to having hardly any infatuation with any maiden and thus turning his heart hairy and his personality cold and untouched by any lady.