When Freida McFadden self-published her first novel, “The Devil Wears Scrubs,” more than a decade ago, she figured it would mark both the start and the end of her literary career.

McFadden, a doctor who treats brain disorders, had a demanding day job, and was raising two small children. But she’d always wanted to write fiction. So to entertain herself at night, she wrote a heavily autobiographical novel about a medical resident who is overworked and humiliated by a domineering supervisor.

“I thought, maybe I’ll publish this book, maybe a thousand people will buy it, and I’ll be done, end of my author story,” McFadden said from her home outside Boston, where she lives with her husband, an engineer; their two children, now 13 and 17; and a cat named Ivy.

“That did not happen,” she added.

Eleven years, 23 books and more than 6 million copies later, McFadden has become a seemingly permanent fixture on the best-seller list. She is currently the top-selling thriller writer in the United States, beating brand names like James Patterson, David Baldacci and John Grisham so far this year, according to Circana BookScan.

Her addictive psychological thrillers are plastered across Amazon’s best seller rankings — on Friday, she held the top spot on the Kindle best-seller list, and had six novels in the top 50. She also sells enormous quantities in print, not only in brick and mortar bookstores, but in grocery and pharmacy chains like Kroger, Aldi and Albertsons.

After a decade of self publishing, McFadden signed a series of deals with Sourcebooks’ mystery and thriller imprint, Poisoned Pen Press, which has acquired print rights to 15 of her books, a mix of new and backlist titles. Since last August, Poisoned Pen has released seven, with two more due out this fall. It’s an unusually packed publication schedule for a single author, but the pace barely satisfies McFadden’s insatiable readers, who call themselves “McFans.”

“Rather than waiting for her to write the next one, we’re peppering in the earlier self-published ones, so her fans are constantly being fed more Freida,” said Paula Amendolara, senior vice president of sales at Sourcebooks.

McFadden’s latest, “The Housemaid Is Watching” — the third installment in her series about a housekeeper who has a terrible secret and even more terrible bosses — came out on June 11 and sold more than 240,000 copies that first week. It debuted at No. 1 on The New York Times trade paperback best-seller list, where McFadden has three books in the top 10.

“The growth has been just explosive,” Shannon DeVito, director of books at Barnes & Noble, said of McFadden’s trajectory. “She’s now a household name.”

As ubiquitous as her books have become, the woman who writes them remains a bit of an enigma. McFadden is a pen name; she is careful not to reveal her identity, mainly because, as a practicing physician, she worries her patients might feel weird about being treated by a best-selling thriller writer.

“At work, I want to be a doctor,” she said. “A lot of my books have medical stuff in them, and I don’t want people saying, ‘Is this based on me?’ It feels unprofessional.”

McFadden is flattered but also flustered by the attention her books have generated. She avoids in-person author events, in part to maintain her anonymity, and finds video calls and interviews unnerving.

“Any situation that’s not my typical routine makes me nervous,” she said. “I could go to work and talk to a dozen new patients and that’s fine, but in a situation where people are like, ‘Oh my God, it’s Freida,’ that absolutely terrifies me.”

Even a recent Zoom event with librarians and library patrons had her spiraling. “What if I have a coughing fit? What if I have a nosebleed?” she said during a phone call the day before the Zoom, sounding panicked. (It went fine.)

Growing up in Midtown Manhattan, McFadden was a brainy kid, a math team member who loved reading and filled up composition notebooks with stories. Her father, a psychiatrist, constantly pressed literary fiction into her hands, while her mother, a podiatrist, introduced her to suspense and thriller writers like Mary Higgins Clark and Robin Cook.

As a student at Harvard, McFadden toyed with becoming a mathematician, but decided to pursue medicine.

During med school and residency, she kept a private blog chronicling the horrors and indignities of life in the medical field. Writing felt like an escape from the pressures of juggling her medical practice and parenting duties, so she decided to try fiction, and repurposed some of her med school stories in her debut, “The Devil Wears Scrubs.” When it sold a few thousand copies, far more than she expected, she realized there might be a market for “medical-ish women’s fiction.”

She moved into medical and psychological thrillers, publishing five more books in quick succession. With novels like “The Wife Upstairs” and “The Perfect Son,” McFadden cemented her brand: thrillers with relatable female characters who often have mundane jobs and prosaic problems — a horrible boss, an annoying colleague, fertility issues, a loveless marriage — but end up, many unforeseen plot turns later, in life-or-death situations.

In 2019, McFadden published her breakout novel, “The Ex,” about a woman who is tormented by her boyfriend’s psychotic ex-girlfriend. McFadden loaded it with as many twists as she could, building to a mind-bending climax that confounded many readers, including her mother.

“My mom loved the book, still doesn’t get the ending to this day,” she said.

After readers complained that the ending was confusing, McFadden rewrote the novel’s conclusion and republished it, bringing up the book’s average rating from 4.1 to 4.2 stars, she noted with satisfaction.

“I learned that it can be too twisty,” she said.

All along, McFadden was working as a doctor, leading a split life. She figured she would eventually run out of plots and quit writing, but the ideas kept coming.

In 2019, she came up with “The Housemaid,” a novel about a desperate woman named Millie who is living in her car, unable to find work because of her criminal past. Millie is thrilled to land a job as a live-in maid for a wealthy family on Long Island, until she discovers that her bosses’ seemingly perfect life and marriage are a facade.

McFadden almost didn’t publish it.

“I thought, I don’t know if this is on brand for me, it’s a little too dark, so I shelved it,” she said.

A few years later, when an ebook publisher, Bookouture, approached her and offered to publish one of her books and promote it on their mailing list, she agreed to sell them “The Housemaid.”

In the spring of 2022, the book became a monster hit. It went on to sell more than two million copies, and was optioned for film by Lionsgate. It has been on Amazon’s best-seller list for 83 weeks, and has spent 60 weeks on The New York Times trade paperback best-seller list.

“The Housemaid” drove hordes of readers to McFadden’s earlier novels. Though she’d mastered digital marketing as a self-published author, McFadden felt she was missing readers who shop in bookstores or pick up books in airports and Target. She signed with an agent in late 2022, and started looking for a publisher to distribute her books in print.

“She had already established this very intimate community of rabid fans,” said Christina Hogrebe, McFadden’s agent. “One of the challenges we faced was finding a publishing partner who shared our vision for Freida’s success and wouldn’t automatically seize the most valuable parts of her publishing platform.”

Since McFadden had already sold so many books on her own, she had an unusual degree of leverage in negotiations with publishers, and struck a deal with Poisoned Pen Press that allowed her to keep her ebook and audiobook rights.

McFadden’s total sales are difficult to calculate, because many readers get her books through Amazon’s Kindle Unlimited, an ebook subscription service that pays self-published authors based on pages read rather than copies sold. Kindle Unlimited accounts for 60 percent of her revenue, McFadden said. Even without that platform, her sales are staggering. In ebook and audio, McFadden has sold more than 3.6 million copies, she said, while her print sales total three million copies, according to Circana BookScan.

As her audience has exploded, McFadden has been hounded by fans who want her to hold book signings. Devout McFans think of her as a friend — albeit the kind who won’t meet in person or tell you their real name — and often invite her to attend their weddings, graduations and bachelorette parties.

“I haven’t received bachelorette party invitations for any other author,” said Hogrebe, her agent.

McFadden finds her success surreal and hard to process. She’s largely stepped away from her medical practice to focus on writing, but continues to see patients once or twice a week in case she decides to return to medicine full time, she said.

Her best-seller status has also brought more scrutiny, including criticism that her books are formulaic and overrated. Some readers complained about a scene in her novel, “The Teacher,” in which a woman finds herself alone in the dark with a possible attacker but without her cellphone because she is wearing a dress without pockets; irate readers griped that McFadden’s description was unrealistic because some dresses do have pockets. Others were upset by the premise of the novel, which centers on an affair between a high school English teacher and his student.

“It’s hard, with everything you do being a little bit more under the microscope, somebody will always seize on something and get mad,” McFadden said.

She mostly shrugs off the criticism and moves on to the next idea.

“I’m just trying to be entertaining,” she said. “I’m not trying to write ‘War and Peace’.”