The Book of Love was a metaphor for an imaginary and literally non-existent list of rules, tips and guidelines for romantic relationships. The so-called Book of Love was often referred to by those asking advice or when exercising their rights and/or privileges within a relationship.
Lieutenant jg Jenna D'Sora referred to chapter four, paragraph seventeen of the Book of Love; "When your girlfriend arrives with a gift, stop whatever it is you're doing, and give her your undivided attention." in the hopes of educating Lieutenant Commander Data on thoughtfulness and courtesy in a romantic relationship. (TNG: "In Theory")