The Heart PrincipleThe Heart Principle is a 2021 romance novel written by Helen Hoang.PlotA professional violinist, Anna Sun, learns that her boyfriend Julian wants an open relationship. Anna then creates a profile on a dating app and meets Quan Diep, a CEO of a clothing company she ends up having an affair with.Critical receptionKirkus Reviews said in its review, "Genre readers will have to judge for themselves if the romance plot satisfies, but those desperate for fiction that explores the crushing weight of caregiving will find it here."