Two sisters who guard the woods that separate their town from fairyland are courted by their power-hungry neighbor in the Hugo Award winner's The River Has Roots (Tordotcom, Mar.).

What inspired the unbreakable bond between sisters Esther and Ysabel?

There’s a type of murder ballad—the cruel sister type of ballad like Loreena McKennitt’s “The Bonny Swans” or Emily Portman’s “The Two Sisters”—that has at its core, as many folktales do, a really antagonistic relationship between sisters. The eldest is jealous of the youngest, and they’re both being courted by the same man, who is somehow not the villain of the song. This always bothered me, because I loved the songs, but the sister story line never worked for me. I’m the eldest of four, and my sister, who is two years younger than me, is my favorite person. She’s the best human being on this planet, without any hyperbole. So, at its core, this story is an autobiographical correction to a 17th-century ballad type.

Why set the story in the woods that border Faerie?

I’m really interested in borders and borderlands. They’re frustrating and difficult, and usually the sites of terrible power being exerted in terrible ways, but they’re narratively very rich. Setting the story in Faerie itself would have meant having a vision of Faerie, which is all well and good. But what was interesting to me was to keep Faerie as sort of an incomprehensible place that we can only glimpse sideways.

How did you develop the magic system, called Grammar?

I love the idea of magic being Grammar, because the order in which you place words, the way that you organize thoughts, all has to do with the exertion of your will in the world. We see that a lot when there’s a story in which someone makes a request of a faerie or gets a boon. The wording of their wish has to be precise so that it can’t be taken wrong or twisted. If you’ve ever been taught to code switch or “talk proper,” you realize that there are registers of English that are more powerful than others. The grammar of a king is different from the grammar of a poet. And the grammar of a ballad is different, both literally and metaphorically, from the grammar of an essay. I wanted to make room for the fact that all of these things have the capacity to be magic.

How did you approach writing Esther’s fae love interest, Rin?

I wanted them to be strange and alien, but also attractive and recognizable as someone you could fall in love with. Whenever a nonbinary gender gets assigned to an otherworldly character, I always have this moment of pause where I’m like, nonbinary people really exist. And I want to make sure that’s acknowledged. It’s fine to use singular they. There still exist, unfortunately, people in my life who are like, “It’s not grammatical.” Listen, I have a lot of Grammar in this book—it is extremely grammatical.