When R.J. Palacio’s book “Wonder” hit the bestseller list soon after its release in 2012, the Park Slope-based writer celebrated for an evening with her husband and two sons.

“That was my ‘Holy crap!’ moment. My son was 7 at the time, and he asked why I was so happy,” Palacio tells The Post. “It was because my book had hit the bestseller list, and when it fell off the next week, at least I could say I’d been a bestselling author for one week. For me, it could not get better.”

If only she’d known. “Wonder” has remained a bestseller for the past seven years, while selling more than 16 million copies and becoming a certified cultural phenomenon.

Unless you write about a certain boy wizard or you’re the Bible, it’s a nearly impossible feat to sell that many books these days. But something connected about Palacio’s story about a disfigured boy named Auggie trying to survive the fifth grade. “ ‘Wonder’ came out when people wanted to hear more about kindness,” Palacio says.

The book’s seed was planted by a real-life incident in which the author’s then-3-year-old son got upset after seeing a girl with a disfigured face at an ice cream parlor. And while the book is aimed at young readers, it apparently has fans of all ages.

“When my older son started college, we met his roommate and his parents and we all had dinner together,” the author says. “The mom asked what I did, and I said, ‘Oh, I wrote a book called ‘Wonder.’ She said, ‘Wait! That’s you?!’ That’s the moment when I realized that it had a readership beyond fifth-graders.”

Now Palacio is releasing a new graphic novel — her first — tangentially connected to “Wonder” and its 2015 companion book, “Auggie & Me: Three Wonder Stories.”

“White Bird: A Wonder Story” (Knopf Books for Young Readers, out Oct. 1) begins with Julian, the bully from “Wonder,” FaceTiming his “Grandmère” for help with a school project. Grandmère reluctantly reveals the story of her time as a young Jewish girl in Nazi-occupied France during World War II and how she was forced to hide in a barn for more than a year. Her only joy came from the visits from a local boy crippled by polio.

Palacio grew up in Queens and was always interested in art as well as writing.

“I realized early on that I could draw. If I wanted to draw a horse, I could draw a horse. It’s kind of like a superpower,” she says. “I loved my brother’s comic books but also Michelangelo. When I was 9 or 10, I’m sitting there copying Michelangelo.”

She attended Manhattan’s High School of Art & Design before majoring in illustration at the Parsons School of Design. Palacio later worked for a book publisher designing covers for Paul Auster, Thomas Pynchon and many others. She only quit her day job three years after “Wonder” came out.

“White Bird” took her two years to complete. She illustrated the book on an iPad with a stylus (Kevin Czap inked it), and Palacio spent more than a year coloring it herself. “I wanted to work with a lot of light and shadow,” the author says. “There’s a lot of light and darkness in people. I wanted to convey that graphically.”

Palacio was halfway through a new prose novel but abandoned it after the 2016 election in favor of “White Bird,” which she hopes is more subtly politically resonant.

“I wanted to be able to introduce the themes of the Holocaust in an age-appropriate way, especially given the times we’re in and the rhetoric that kids are exposed to every day,” she says. “They hear this administration using words like bans and referring to people as infestations. From the moment I heard these words, I was deeply troubled.”

Although “White Bird” takes place in the “Wonder”-verse, no prior knowledge of the characters is necessary for new readers.

Palacio has several ideas for new projects and says this may be it for Auggie and his gang. “I’m ready to move on from the world of ‘Wonder,’ ” she says.

The book has already brought her a new career, international acclaim and something else.

“Having grown up as the daughter of [Colombian] immigrants in a one-bedroom apartment, money has been a huge issue,” Palacio says. “It’s nice being able to relax about how to pay for college.”