Horan began “an exploration of an amazing pair of people.” The relationship spanned two decades and three continents, and left plenty of material. “Stevenson alone, eight volumes of letters!” she says, laughing. But Horan’s careful process (the book was five years in the making) relies on the historical context the documents provide.

“That’s what really draws me to a story. I like it if there’s an engaging series of events and a change in the characters. I begin by doing research, but it continues all the way through, and it’s absolutely the case with this story. I know where it’s going, I have a general idea of what they were doing in a given year and what their lives were like, but during that process there’s so much to read and so much to learn.” It doesn’t all find a place (“Chapters will bite the dust,” she says pragmatically), but the background “enriches the whole process.”

Horan is no stranger to reconstructing historical figures; her 2007 blockbuster debut, Loving Frank, took Frank Lloyd Wright’s mistress, Mamah Borthwick Cheney, from historical footnote to heroine of a novel about identity and the public eye. That approach brought Osbourne’s voice to the fore in Under the Wide and Starry Sky. Though their stories differ, Horan sees both women in conversation within the genre of historical fiction. “You find they were up against challenges that we don’t necessarily face any more.” In particular, “The issue of divorce was alive for both of those women, and a very hard obstacle to overcome.”

Despite being well-regarded among his contemporaries, Stevenson’s writing fell out of favor in the 20th century. However, he’s enjoying a critical renaissance, and Horan hopes that readers might come back to Stevenson’s work with a new eye.

But historical and literary significance aside, she considers the novel a personal journey. “I hope that people are as engaged and captivated by these people as I was,” she says, with the passion and familiarity one assigns to old friends.

Horan signs today at 11 a.m. at the Random House booth (2739).