Jeannette Walls says the first time she read the finished version of her bestselling memoir, The Glass Castle (Scribner, 2006), “I was like, 'Dang, I got a weird life! Nobody's going to be able to relate to this! Everybody's going to think I'm just a poor white trash loser.' But the shocking—and gratifying—thing was how many people have understood what I was trying to say.”

Glass Castle is about Walls's childhood; her father was an alcoholic and her mother was a nonconformist, to put it mildly. Walls, her parents and her three siblings spent years living in rundown Arizona desert towns and then in a West Virginia shack without plumbing or electricity. Her parents became homeless; the children pulled together, saving to send the eldest sister on a bus to New York City. Walls followed after high school and eventually all the children made their way to New York. Walls graduated from Barnard College and became a gossip columnist for MSNBC. Glass Castle has sold more than 2.5 million copies, and Walls, 49, has toured the country, meeting thousands of readers. She says, “I cannot tell you how many people came up to me and said, 'The details of our lives are very different, but you and I have a lot in common.' I think that's why people read memoirs—for this emotional connection.” Walls connected with readers so intensely, they provided the inspiration for her next book, Half Broke Horses, which Scribner will publish with a 500,000-copy first printing.

Readers told Walls they wanted to know more about her mother. So Walls started interviewing her mom, who now lives with Walls and her husband in Virginia (her father died toward the end of Glass Castle). But as Walls probed for details, her mother told her, “This book should not be about me. It should be about my mother.” Walls had always been enamored of her grandmother, who died when Walls was eight but left vivid memories. “She was always cursing and dancing, whipping out her gun or whipping out her teeth,” Walls recalls. She liked the idea of writing about a woman who'd lived through droughts, floods, tornadoes and the Great Depression. “People have gone through much worse than I ever did,” she says. “I wanted a good story of somebody who'd gone through some rough times.” Lily Casey Smith fit the bill.

Lily's story was fascinating: born in 1901, she grew up in west Texas, learning how to break horses. She left home at 15 to teach in a frontier town 500 miles away, and later moved to Chicago. She returned to the country and ran an Arizona ranch with her husband, learned to fly a plane and raised two children. Walls found it easy to write in Lily's voice—a resilient, determined voice not unlike Walls's own in Glass Castle. She figured she'd write from Lily's point of view, and then rewrite it into third person, adding phrases like “my understanding was,” or “I believe it was.”

Walls's agent, Jennifer Rudolph Walsh, and editor, Nan Graham, convinced her the story was more immediate in Lily's voice. But how could Walls write a nonfiction biography of Lily drawing only on her and her mother's memories? Although Walls corroborated everything she could with family members and books about Lily's grandfather like Robert Casey and the Ranch on the Rio Hondo by James Shinkle, she didn't feel honest calling the book nonfiction. “Once you start assuming or plugging up holes, jumping to conclusions, it's no longer pure. Once it's no longer completely nonfiction, then it becomes fiction.” So Half Broke Horses' subtitle is A True-Life Novel.

Walls's hope for the book is that it inspires readers to examine their own family histories. “We all have these really interesting ancestors. Some people have really tough stories,” she says.

People ask Walls how she could forgive her parents. But for her, it's more about acceptance and understanding. “We all have our baggage. I think that by looking at somebody's story, the answer becomes clear. That's why I've always been such a big fan of nonfiction, because the answer's always there, if you're willing to dig deeply enough. You might not love the answer. But if you're willing to look and unearth things, you can understand people a little better.”