The Heart Principle

Two years ago, I fell in love with The Kiss Quotient and The Bride Test, the first two books in Helen Hoang’s Kiss Quotient trilogy. Ever since I read those two back-to-back, I’ve been desperate to read The Heart Principle, and it was well worth the wait: It’s my favorite in the series. This is a book I’ll be urging everyone I can to read.

Although it doesn’t officially publish until August 31st, Book of the Month offered The Heart Principle a tad early. (They also have the first two books, so you can get the whole series as a matching set!)

The Heart Principle by Helen Hoang
SeriesThe Kiss Quotient (#3)
GenreRomance; Drama
SettingCalifornia, United States
Number of Pages331
Format I ReadHardcover
Original Publication DateAugust 31, 2021

Official Summary

When violinist Anna Sun accidentally achieves career success with a viral YouTube video, she finds herself incapacitated and burned out from her attempts to replicate that moment. And when her longtime boyfriend announces he wants an open relationship before making a final commitment, a hurt and angry Anna decides that if he wants an open relationship, then she does, too. Translation: She’s going to embark on a string of one-night stands. The more unacceptable the men, the better.

That’s where tattooed, motorcycle-riding Quan Diep comes in. Their first attempt at a one-night stand fails, as does their second, and their third, because being with Quan is more than sex—he accepts Anna on an unconditional level that she has just started to understand herself. However, when tragedy strikes Anna’s family she takes on a role that she is ill-suited for, until the burden of expectations threatens to destroy her. Anna and Quan have to fight for their chance at love, but to do that, they also have to fight for themselves.


The Heart Principle was one of my most anticipated books of 2020 and, once it was pushed back a year, of 2021. As such, I couldn’t wait a day longer when it arrived in its blue Book of the Month box two days ago: I started it right away, on a Thursday night. Now it’s Saturday afternoon, my heart is full after reading it, and I’m feeling so much right now as I write this review.

Like the previous two books in the series, The Heart Principle is a romance, but it moves away from the light and comedic tone of its predecessors. Helen Hoang did warn that this would be heavier and deal with weightier themes – including mental illness and grief – but in my opinion, that makes The Heart Principle even stronger.

The story opens with Anna Sun, a talented violinist who is stuck in a cycle, unable to finish playing any piece of music. It’s been ongoing for six months, and she’s in therapy to overcome it, but she’s not yet in a place to get out of that cycle. And though she’s hidden it well for her whole life, a professional diagnosis of being on the autism spectrum both shocks Anna and helps her understand herself in a way she never has.

Neither her family nor her longterm boyfriend, Julian, seem to realize how much Anna masks and hides and changes herself in order to please them. But when Julian wants to open up their relationship, Anna figures a one-night-stand would be a perfect opportunity for her to practice being authentic and asking for what she truly wants.

Then there’s Quan Diep: He was a lovable character in This Kiss Quotient and The Bride Test – the older brother of Khai and cousin/best friend/business partner of Michael. He’s lost a bit of swagger since then, though: He’s just overcome a battle with cancer, and his self-esteem has taken a hit. Like Anna, he’s looking for a safe place to recover a bit of himself, and a casual hookup seems like the right option.

Throughout the full novel, The Heart Principle has so much heart and tenderness. Both Anna and Quan are working through personal struggles, and that unites them in a special way. They understand each other and are willing to keep trying for each other. Quan, in particular, takes on a nurturing role that ends up being a lifeline for Anna when she reaches new low points.

I love how The Heart Principle tackles such difficult themes, showing the dark and ugly sides of things with the kind of depth usually reserved for literary fiction rather than romance. One major theme that takes up a large part of the book is how exhausting it can be to serve as a caregiver to a loved one. This can be hard on anyone, especially when it lasts for months on end. The situation is made even harder for Anna because of how little her family understands her. Her sister Priscilla, in particular, is overly critical, cold, and devoid of compassion for Anna. She refuses to acknowledge Anna’s autism, her personal caregiving limits, and her deteriorating mental health.

It’s painful to see how much Anna wilts in her family’s presence and how little they express any care for her. Even gratitude and compliments are rare from her sister and mother. This is what makes Quan stand out so much. In contrast to the aloofness of her family, Quan is always kind, caring, and patient with Anna – from their first (failed) attempt at a one-night-stand together to the darkest days of Anna’s eventual autistic burnout.

The Heart Principle offers many valuable lessons. Who truly understands you and values you, and who just likes it when you do what they want? How can constantly appeasing others ultimately tear you down? How can you learn to say “no” to people, set your own boundaries, and put yourself – and your health – first? For all the struggles she faces, Anna has inner strength that gets her through. But we also see how being with someone who loves you can help immensely. Quan is a someone she can depend on, someone who loves her at worst and wants to see her feel her best.

I love love love The Heart Principle for its nuanced and honest depictions of mental illness, caregiving, autism, burnout, the aftermath of surviving an illness, therapy, suicidal ideation, and complicated family issues. But I also love some of the lighter elements here, too. As a lifelong music fan, I enjoyed seeing an accomplished violinist as a protagonist – including the darker side of fame and being paralyzed by fear of criticism. I also loved seeing two characters that seem so different on the outside – one a tattooed, motorcycle-riding, highly athletic cinnamon roll, the other a classically trained performer in the San Francisco Symphony with a record deal – but they’re so well matched on the inside. They’re a strong team, able to support each other through thick and thin.

As I finished The Heart Principle, it struck me that in addition to being a love story between two characters, it’s also a self-love story. We all need to put ourselves first sometimes, even if that means disappointing others. We all need to take care of ourselves, no matter the surrounding circumstances, and we all deserve to be understood, respected, and loved for who we are. This is as much a romance as it is a heartfelt journey to overcoming mental health struggles and finding yourself again. For all the darkness Anna and Quan face, there is a light at the end, a happily ever after to be found, even if it will always take effort and courage.

Final Thoughts

The Heart Principle is a beautiful, heart-wrenching, and inspiring book that will speak to anyone who has faced a mental health struggle, burnout (including autistic and caregiver burnout), or difficulty speaking up for yourself. It’s filled with love and, ultimately, gentle empowerment. As she states in the Author’s Note at the end, The Heart Principle is a very personal book that dives into circumstances that Helen Hoang herself faced. It’s “half memoir,” and that only adds to how impactful this novel is. It’s a perfect end to the Kiss Quotient trilogy, and the best book from Helen Hoang yet. I greatly look forward to reading more from her in the years to come.

Rating: 5 out of 5.

About the Author

Credit: Eric Kieu

Helen Hoang is that shy person who never talks. Until she does. And the worst things fly out of her mouth. She read her first romance novel in eighth grade and has been addicted ever since.

In 2016, she was diagnosed with Autism Spectrum Disorder in line with what was previously known as Asperger’s Syndrome. Her journey inspired THE KISS QUOTIENT.

She currently lives in San Diego, California with her husband, two kids, and pet fish.

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