Sixty thousand is the number of copies Christina Baker Kline thought her newest book, Orphan Train, might sell in her “wildest dreams.” For a midlist author on her fifth novel, it was a lofty number. It turns out, it was also way too low.

After being published by William Morrow on April 2, 2013, the paperback original has spent 30 weeks on the New York Times trade paperback bestseller list, hitting #1 last week. According to its publisher, the book has sold more than 815,000 copies (in print and digital combined), over 21 printings. Now Morrow, looking to take advantage of this moment, is repackaging all of Kline’s backlist novels and will begin reissuing them this summer. The publisher is also trying to take stock, and figure out what went so right this time around.

Orphan Train’s success, according to both Morrow and Kline, ultimately came down to three things: a change in format, a change in subject matter, and a key retailer promotion.

Morrow has published all of Kline’s fiction—she has written four nonfiction titles for other houses—and, to date, she has never reached the kind of numbers she’s hitting with Orphan Train. Morrow declined to provide Kline’s sales history, but described her as a “well-received” midlist author. According to Nielsen BookScan, which tracks about 80% of print sales, one of Kline’s bestselling titles with Morrow before Orphan Train is 2007’s The Way Life Should Be, which sold just under 26,000 copies in paperback and hardcover combined. Her last novel, 2009’s Bird in Hand, sold, per BookScan, just under 5,500 copies in paperback and hardcover combined. For Kline’s editor, Kate Nintzel, the impact of the publisher’s decision to release Orphan Train as a paperback original, rather than in hardcover, cannot be underplayed.

Unlike Kline’s previous novels, which did not feature historical elements, Orphan Train touches on a footnote in American history. The novel, which moves between the present and the past, follows the relationship between a teenager named Molly, who is part Native American and is aging out of the foster care system. When Molly meets an elderly woman named Vivian and learns her history—as a young Irish immigrant in New York, Vivian was shipped off on a so-called orphan train, with hundreds of other unclaimed children, to the Midwest—their surprisingly similar stories mark the beginning of an unlikely friendship.

Nintzel thinks the “historical angle” in this novel brought her author an entirely new readership. Kline agreed. “This novel is about a piece of American history that’s pretty important, and has been hidden in plain sight,” she said. Many of Kline’s new readers, it turned out, wanted to feel like they were educating themselves, whether they were diving into fiction or nonfiction. Kline elaborated: “There’s a whole world of people who read fiction as a way to learn, [and I’ve] never tapped into this before.”

On the publishing side, Nintzel thinks the decision to release the book as a paperback original made it more book club friendly and, perhaps inadvertently, led to one of the most important pick-up’s the title received. Target chose Orphan Train as one of its Club Picks (the promotion features a collection of titles the retailer arguably gears toward book clubs, and it is open only to paperbacks), and this, Nintzel said, “helped launch [the novel] in a much bigger way.” After the Target selection, Morrow amped up its publicity and marketing efforts, and the book has, according to Nintzel, sold steadily since it came out. (Orphan Train landed on the New York Times’ extended bestseller list, for paperback fiction, in its first week on sale.)

That Kline has tirelessly promoted the novel since publication, often in bookstore and library appearances, has also helped. Kline has done over 70 events at bookstores and libraries alone; she has also, Morrow estimated, spoken at some 20 schools. Kline “has been an incredible part of why [this book] has worked,” Nintzel said. “She’s been working so hard, and doing so many different types of events.”

The touring, the editor believes, has continued to push word of mouth. And, because Kline approaches her talks as opportunities to teach about the orphan trains (at many events she uses a PowerPoint presentation), she has become an in-demand speaker well beyond the traditional bookstore circuit.

“There are so many ways to talk about this book,” Kline said, distinguishing Orphan Train from her previous novels. Not only is she seeing more men at her events—in the past she drew largely female crowds to her readings—but she’s hearing that her readers appreciate this kind of book. Noting that her last novel, Bird in Hand, was about a suburban marriage, Kline recalls an exchange that happened with a fan, while she was touring for Orphan Train: “She said, ‘I am never again going to read a story about a suburban marriage.’ She told me she wanted to read about characters and history; people who are different from her.”