Husband-and-wife writing partners Nicci Gerrard and Sean French had already written a dozen standalone mystery novels under the pseudonym Nicci French when, in 2011, they created Dr. Frieda Klein, a character so complex that she demanded a series of her own. Five years later the successful series, with each entry named for a day of the week, continues with its fifth title, Friday on My Mind (Penguin Books, Oct.), in which Frieda is accused of murder after the body of her ex-boyfriend turns up in the River Thames.

French, who shares a home outside of London with Gerrard (they married in 1990), says: “We always wrote standalone books with female central characters. They were sort of ordinary people who wouldn’t be constantly involved in solving crimes. But Frieda, a psychotherapist, came to visit us in a way.” French and Gerrard had an idea about a woman who doesn’t want to be a detective but who is cursed with the gift of being very good at discovering others’ secrets and following clues that ultimately solve a range of brutal crimes.

“The therapist discovers the solution to someone’s problems much in the way a detective works at following clues,” French says. “Frieda doesn’t believe you can solve the outside crimes of the world unless you address the psychological issues inside people.”

At the beginning of the series, Frieda is adept at guarding her own secrets. Gradually, though, Gerrard and French reveal the inner life of this prickly, difficult, but brilliant woman, one layer at a time. “We needed a series, because Frieda wouldn’t disclose herself in one book,” French says. “And we named the books after days of the week because we want to see the passing of time and Frieda’s growth as a person.” All the characters in the series change along with her from book to book and are marked by what they’ve gone through as participants in Frieda’s sleuthing.

One such regular, a Ukrainian handyman named Josef, crashes through the ceiling in Frieda’s consulting room and lands at her feet. He survives the fall and ends up working for Frieda, becoming her friend in a most unlikely alliance. In fact, he becomes one of the very few people in Frieda’s life that she trusts. “We didn’t know if Josef would be around for the whole series, but he wouldn’t go away, so he stayed,” French says.

Such a scene was actually witnessed by the author couple about 25 years ago, Gerrard says: “We were having lunch with friends in their house, and a worker came through the ceiling and landed on the floor with a crash. Of course we thought he was dead, but he got up, wiped his hands, and walked away.” She and French laugh over this memory in a display of their easy, devoted relationship; they so clearly enjoy each other’s company.

“Maybe Frieda is a counterpart of what we do as writers, sitting alone and working as a therapist without having to leave that space of control and safety to go out into the messy, dangerous world,” Gerrard says. French adds that a therapist can give shape to a life, just as a writer does: “Real life doesn’t have a narrative, but therapists and writers might provide that narrative. To Frieda, our moral duty is to face up to the truth, but what our books show is that facing up to the truth doesn’t guarantee any kind of happy ending.”

Gerrard and French are both in their late 50s and both attended Oxford University. They come from distinguished backgrounds in journalism. She worked at the Observer for many years, while he was at the New Statesman. “I started out on kind of the literary desk, and by the end of my time there I was doing big stories,” Gerrard says. She wrote about the dark side of English society, for example the story of notorious serial killers Rosemary West and Fred, her husband. “I was known at the Observer as the correspondent of sorrow,” Gerrard says, laughing, “But it was an extraordinary privilege to witness people’s lives in a way not many of us have.”

“What was especially shocking about the young women killed by the Wests was that nobody knew they were missing,” French says. He adds: “This has haunted us for a long time. They weren’t even mourned or missed. This is the thread that runs through all our Nicci French novels, the many lives that go unnoticed. Frieda believes that all life is precious, and you don’t need to be rich and famous and beautiful. When we write about the crimes and the deaths in the series, we always have that reference.”

After 20 books in as many years, the pair acknowledge that their cowriting technique springs from a place of harmony and trust. “We plan a book together in great detail,” Gerrard says. “We work on the plot and the kinds of feelings we want to generate and the tone of voice.” The couple also shares in the research a particular book might require. “We go through cups and cups of coffee and tea, and then glasses of wine, over a period of time,” Gerrard explains. “When we know we’ve both got the same book in our heads, then we start writing. One of us will write the first chapter and email it to the other, who is theoretically completely free to add to it, to edit it, to erase it, even. Then it gets emailed back. So it passes back and forth between us.” They claim to be very different kinds of writers, with different interests and imaginations, but Gerrard emphasizes that in their collaborations they are not writing as Nicci Gerrard and Sean French, but as a third person: Nicci French.

Gerrard and French adhere to a strict set of rules when they’re writing a book. “The first is that we’ll never tell anyone who wrote which bit,” Gerrard says. “It’s a recognition of the fact that just by approving of something we’ve written and liking it, there is an absolute sense of joint ownership of every dot and comma.”

The second rule is that they never sit down with each other to work, nor will they criticize each other’s work when they are together. “He will email his chapter back to me, and I’ll change it,” Gerrard says. “But he won’t know what I’ve done until he receives it back and vice versa.”

Because editing each other is such an “intimate and vulnerable” process, maintaining a physical distance while working is essential to Gerrard and French. She works in the attic of their house in much the same way Frieda retires to the garret of her London home; French writes in a garden shed cum office in their backyard.

The third rule, French explains, is that “if, for instance, I notice that Nicci has cut something of mine out, I can’t reinstate it.” He adds: “We trust each other to be each other’s best reader and editor. We do things for good reasons rather than power reasons. We do not argue about the writing. I have never felt these books would be better if I’d written them on my own.” This formula seems to work, because the writing in the Nicci French books is seamless. Most of their standalone books, including Beneath the Skin (2000), Losing You (2006), and Complicit (2009), have been bestsellers.

The couple is adamant that there will only be eight books in the Frieda Klein series. “We were very clear about this, right from the beginning,” French says. “There’s a certain deliberate shape to the Frieda books that brings the series to a logical end.” Friday on My Mind will be followed by Saturday Requiem, already completed, in 2017. They’re writing the Sunday book now, and finally there will be, according to French, “a mysterious eighth day—and after eight days, Frieda would have had enough of us!”

Despite the appearance of a perfect marriage, Gerrard says they’re “like everyone else”: “We bicker and row. We deal with irritating habits, like dirty socks and washing up. But we never argue about the writing.” And how do they keep the writing fresh after so many years as collaborators? “The work stays interesting for us because we keep pushing each other in a way we normally wouldn’t and couldn’t manage on our own,” French says. Gerrard has a similar perspective on this. “One of the fascinating things about our partnership is the way we write for each other, trying to surprise each other, shock each other.”

With the Frieda Klein series coming to a close, and after immersing themselves in the world of this enigmatic, complex character for several years, Gerrard and French are looking forward to taking a break. “We have one plan,” French says. “We’re going to go away, somewhere remote where we can take long, lonely, beautiful walks, and we’re going to talk about where we’re going with our next writing project.”