Angie Kim was on her fourth career—lawyer, consultant, entrepreneur, and then stay-at-home mom—when she took up writing in her 40s. Number five was the charm, it seems. Kim’s 2019 debut novel, Miracle Creek—about a murder trial following the explosion of a hyperbaric oxygen therapy unit with parents and children inside—was translated into 20 languages, became a national and international bestseller, and won the Edgar Award for best first novel.

Based on the tone of early reviews and the avalanche of prepublication anointments— IndieNext, Book of the Month, Library Reads, and more—her second book, Happiness Falls (Hogarth, Aug. 29),  could make Kim a household name. It revolves around a family with a Korean immigrant mom, a white dad, and three biracial kids: hyperintelligent, hyperverbal 20-year-old Mia; her twin brother, John, who is what the kids would call normcore; and her younger brother, Eugene, who has both autism and a rare genetic condition called Angelman Syndrome, which makes him unable to speak.

On the day the story opens, their father, Alan, disappears while on an outing with Eugene, who returns alone, unable to recount what happened. The story of the ensuing search is narrated by Mia, described by our besotted critic as “a brilliant, acerbic chatterbox whose relentless analysis spills from long, ropy sentences into parentheticals and footnotes that lasso the reader into turning the pages.” The book earned Kim a second Kirkus star.

There are many common elements in Happiness Falls and Miracle Creek. Both combine an exploration of neurodiversity and its effects on family dynamics with a mystery, both are set in the Northern Virginia suburbs, and both feature a therapy center called Henry’s House and a brilliant lawyer named Shannon Haug.

We knew Kim was a lawyer, that she lives in Northern Virginia, that her biracial kids have a white dad—but these are just the most superficial of the book’s autobiographical connections. The day we joined her for a conversation on Zoom, she had come directly from Motormorphosis, the flagship conference for the nonspeaking community. She was excited to have participated on panels with nonspeaking authors who were using systems for communication involving letterboards and iPads, much like the method described in her novel. And that is where our conversation began, delving into the profound connection between Kim and her characters. Our exchange has been edited for length and clarity.

How did you become so fascinated by the situation of nonspeakers?

When I came to this country as an 11-year-old [from Korea], I had a really good sense of myself. I felt competent and intelligent and articulate. Then I came here, and, overnight, I couldn’t speak. Though I know I didn’t become stupid overnight, that’s how I felt and how I was treated. That was the beginning of my understanding that our society equates oral fluency with intelligence. Even though it only affected me temporarily—I now speak English without an accent—the insecurity stayed with me. Forty years later, my husband and kids tell me, “You’re the most insecure person we know!”

If that experience is behind the character of Eugene, his sister, Mia, is the polar opposite. As she explains herself, she’s hyperlexic—obsessed with reading since early childhood—and she never shuts up.

Some of that also comes from my life. Growing up in Korea, we were so poor, my parents and I lived in one tiny room—so small we didn’t really have room for furniture or anything. We didn’t even have beds, so we just rolled out these mats every night for sleep. I was obsessed with reading from an early age, so would borrow books from my friends and read them over and over. Then, when I was 10, my parents gave me a beautiful set of classic mystery stories like Poe’s “The Black Cat,” illustrated hardbacks that they found at a flea market that I was allowed to take with me when we moved to America. I read these books so many times that I think mystery became part of my DNA. I still have them right here!

When my aunt gave me her favorite Sidney Sheldon novel, Rage of Angels, I read it to learn English, using a dictionary and writing down lists of words. One of my first published pieces—this was in my 40s—was a humor essay about that experience, which won a Glamour magazine contest judged by Erin Morgenstern, Jane Smiley, and Jesmyn Ward.

That is so cool. Before you took up writing, you were a lawyer, right? That background comes into play in both books through the character of Shannon Haug. In Miracle Creek, she’s a central player in the courtroom drama; in Happiness Falls, she helps the family protect Eugene from accusations regarding his father’s disappearance. Do you see the books as a series?

More as two sides of a coin. While Miracle Creek explored the issue of having a child with disabilities from the parenting perspective, Happiness Falls looks at the situation through the child’s lens, and the sibling’s lens. Also, Miracle Creek had seven different POVcharacters written in close third-person perspective, and this one is very decidedly first person. No matter how tough it was to stay with Mia throughout—it would have been so easy at times to move the plot forward with somebody else’s perspective—I stuck with her. In the end, I felt that staying in Mia’s head reinforces the mystery element, not being able to access anything except what you yourself are thinking. I see the mystery as a Trojan horse that gets the reader pulled in and then gives me the container to hold all the ideas I want the book to explore.

And one of those ideas is the part of philosophy called happiness theory.

Right. Mia finds her father’s notebooks, where he is working on the question: What is happiness? He’s particularly interested in the theory of the relativity of happiness, which proposes that our happiness level is dependent on both our expectations and our baseline.

Mia learns that he’s been secretly running experiments to test this on his family, trying to formulate equations that explain the result. Happiness theory is the heart of the book—the aspect that has caused multiple readers and reviewers to describe the novel as life-changing. And it occurs to me that the notebooks are where we do hear a voice other than Mia’s—her father, Alan’s.

You’re right! You know, they just cast the audiobook, and in addition to the person who plays Mia, there’s an actor who reads the notebooks, and—I’m so excited about this!—a real-life nonspeaker named Tom Pruyn does Eugene.

How is that going to work? In the book you use all caps when we get access to Eugene’s thoughts, or when he communicates by using a letterboard.

Tom is actually what’s called an unreliable speaker: He can’t always predict what will come out of his mouth, and uses a letterboard to communicate. Fortunately, he was able to read the script out loud. At 25, it’s always been his dream to be a performer. This was his first paying gig, and he did amazingly well. At the conference today, the whole nonspeaking community was celebrating for him.

Marion Winik is the host of the NPR podcast The Weekly Reader.