"The Heart Principle" is a contemporary romance novel by Helen Hoang, part of her Kiss Quotient series. The story follows Anna Sun, a violinist who struggles with the pressures of her career and personal life. As she navigates her relationships, Anna discovers the importance of self-acceptance and the complexities of love. The novel explores themes of mental health, family expectations, and the pursuit of passion. Through Anna's journey, readers witness her growth and the challenges she faces, making it a relatable and heartfelt narrative about finding one's true self amidst life's chaos.