British author Daisy Johnson is in her Oxford study on an unseasonably hot day in late May. “Everyone’s kind of piled out into their gardens,” she says, gesturing via Zoom at the glare from the window, which falls on her shoulders and wavy blond hair. She’s kept busy writing during the pandemic, as is evidenced by the gentle sway of Post-it notes on the wall behind her.

Johnson’s first book was a story collection, Fen, which she wrote while pursuing her masters at Oxford and working in a bookstore. A year later, in 2018, she published a novel, Everything Under. Then the unexpected happened. Johnson was babysitting when her editor called with the news that she’d been shortlisted for the Man Booker Prize. “The little girl I was looking after was excited, too, and we went to a playground and ran around and around and around,” she recalls.

The recognition has changed Johnson’s life, allowing her to write full-time and reach a larger audience than she’d imagined. Fortunately, she says, she’d already begun her second novel, Sisters (Riverhead, Aug.), when she found out about the Booker nomination. “It meant that after everything calmed down, I could get back to work.”

Sisters is about July and September, ages 15 and 16, and their depressive mother, Sheela. Like Johnson, they live in Oxford, but after a catastrophic series of incidents that begins with July being cyberbullied, Sheela uproots them.

These events drive the plot, but they are secondary to the psychological horror faced by the family in the isolated seaside cottage, situated at the edge of the moors in northern England, where they move. There, Sheela disappears into her bedroom, leaving the girls to play a grotesque, increasingly dangerous game called September Says, in which July is expected to do anything her sister asks. In one instance, July agrees to eat an entire jar of mayonnaise; in another, she agrees to sacrifice her life in the event that only one of them could survive.

Johnson originally planned to set the book in Wales, but after teaching at the University of York and living with her partner in a converted Ford Transit van—driving around the nearby national park with him and camping—she decided to set it in North Yorkshire. “We were living in quite a small space, both of us trying to work,” she says. Those tight quarters served as inspiration. The region, she adds, is “a very rural, wild area, as much as you can get in the U.K. I wanted nature to surround the characters, which is why there’s a murmuration of birds early on and then a swarm of ants. The idea of the house being kind of inside and outside really interested me.”

Johnson says she’s fascinated by the in-between age before adulthood. “I remember everything feeling very bright and intensified, and the weird superstitions and the nastiness. I went into this book thinking about scary things and monsters. That age in particular is a really good place to explore because you are in this liminal space where it feels like uncanny things could happen.”

At one point, while July and September are messily repainting the walls of the house, a stream of ants comes through a crack in the wall, followed by a noise that sounds like a scream. Then a beak tears through the plaster and a bird emerges, with more ants “digging beneath the down.” The next morning, July wakes and looks for the hole, unsure of whether the episode was a dream, and all she sees are globs of half-dried paint.

Johnson wrote the scene to leave the reader guessing as to whether it was a dream (or hallucination) or it really happened. After hearing that the passage made this reader squirm, she says she’s glad. “I’m pleased you said that because for me, when I was growing up, I really loved reading books where you begin to understand what it means to have a body. There’s a very uncomfortable and slightly squirmy thing about existing as a human with other humans. I didn’t want Sisters to be a horror novel, but I wanted it to take aspects from horror, one of them being this kind of body horror, particularly from a female point of view. They are in a very intense situation, and I wanted it to feel very intense for you, too.”

Johnson was born on Halloween and says her parents gave her lots of horror books. “I immediately loved Stephen King, and grew up feeling very connected to those books. And then when I started thinking about Sisters, I knew I wanted to return to them. I was also reading a lot of Shirley Jackson and some Anne Rivers Siddons.” She has been excited, also, at how the genre has evolved in films over the past decade. “There’s been something really interesting happening, with Babadook and It Follows exploring feminist issues. I’ve loved that way of looking at something from an angle, at big themes through the lens of horror.”

The subject of cyberbullying came late in the writing, Johnson says, partly because she’s generally interested in maintaining a timeless quality in her work. “The more I wrote the characters, the more I realized that things like social media or the internet or having a phone were going to be for them a lot bigger than they were for me. This is a book about hauntings in various ways, and I wanted it to feel like the internet or social media is haunted for them as well, and not necessarily trustworthy.”

Peter, the girls’ father, who died years earlier, and whose sister gave Sheela the cottage so they could leave Oxford, doesn’t literally haunt the characters, but memories and conceptions of him take up a great deal of space in their minds. Sheela remembers her violent fights with Peter and how she’d left him when the girls were babies, seeing him as “a black hole” that would eat them alive. She found fulfillment writing children’s books, using her daughters as subjects. But as the girls grew older, Sheela’s relationship with September soured, taking a heavy toll, especially as Sheela recognized Peter in her. Johnson expresses Sheela’s burden of being a single mother in brutal, lyrical prose: “Her love for them was like carrying shopping bags up a hill and at times she became convinced they wanted the very foundations of her, wanted to break the bricks of her body apart and climb back in.”

Johnson says she worried about writing Sheela, since she is not a mother herself and has a very different relationship with her own mother. The character came from “talking to women and seeing the way women react to grief,” she notes. She wanted to explore what happens when a mother “suffers from heavy depression, which changes her relationship with her children.” She also thought about her own response to “the pressure of having children, and the fight between being a mother and also having a career.”

Everything Under took Johnson four years to write, but Sisters came together more quickly. “I think I trusted and enjoyed my methods, but it changed a lot, and it’s a lot shorter than it was,” she says. Now, rereading it, she’s struck by the difference in the writing compared to her past work. “There’s this breakdown in the language and a different use of space on the page, which I think has come from what the book is about.”