Some years ago, American writer Lionel Shriver was faced with having to pick one partner over another. It was a painful experience—they were each good choices—and ever since, Shriver says, "the other life in which I had made the opposite decision had a funny kind of reality to it. Even years later, there is a parallel universe in my head, of if I hadn't left, and what would that be like?"

Dressed in black capri pants and T-shirt, Shriver sits, barefoot, on her Brooklyn home's enclosed front porch and talks about the inspiration behind her eighth book, The Post-Birthday World, which HarperCollins will publish in March. (She mainly resides in London, but has a place in New York because her husband, a musician, frequently plays here.) The novel's central character, Irina McGovern, is an American children's book illustrator living in London with her longtime boyfriend, think-tank research fellow Lawrence Trainer. Irina and Lawrence have a routine but loving relationship.

Every year they take their friend Ramsey Acton to dinner for his birthday, and this year Lawrence is away on business, so Irina halfheartedly dines alone with Ramsey. The recently divorced professional snooker player turns on the charm, gets Irina drunk, and the two almost kiss. An intense alternating parallel narrative follows: in one, Irina imagines her life as she gives in to temptation; in the other, she stays with Lawrence while fantasizing about Ramsey.

"I, like Irina, have been in a position of choosing between two men," Shriver says. "This [book] is the product of having made the decision and being haunted by having taken the other fork in the road." The author particularly wonders how her recent career transformation might have played out with the man she didn't choose.

Up until 2003, Shriver, who has lived in the U.K. for nearly 20 years, referred to her career as "a 25-year slog." She'd published six novels in a row that didn't earn out, except, she says, "as an object lesson in how easy it is to squander other people's money." But things changed with her seventh novel, We Need to Talk About Kevin. The book was a breakout, critically and commercially. It won the Orange Prize and, according to Nielsen BookScan, has sold 44,000 copies in the U.S.; while in the U.K., Shriver says, it just crossed the half-million mark. With one book, Shriver "went from being a nobody to a semisomebody." Suddenly, her readings were packed. She traveled to international events. She was barraged with media requests to talk about Kevin—which is written from the perspective of a mother whose teenage son has killed nine people at his school. And Kevin's success didn't just alter Shriver's public life. It caused her to think about how her life might have been different had she chosen the other partner, and wound up providing inspiration for Birthday.

Living in the U.K. gives Shriver (born and raised in North Carolina) "cultural dual citizenship," a trump card she has used in her novels as well as in her journalism (she writes about politics and culture for the Wall Street Journal, The Guardianand The Independent). Shriver admits to feeling overwhelmed by the competition in the U.S.—"being another fiction writer who lives in New York, Brooklyn especially, is not special"—and finds she fills a niche market abroad, interpreting the Americans to the British and vice versa. Eventually, she says, she'll move back to the U.S., where her fan base seems to be growing (Harper Perennial will release two of Shriver's backlist titles, Game Controland A Perfectly Good Family,in paperback in July). But Shriver hopes to forestall her return "as long as possible, because I am convinced that if and when I do, my world will get considerably smaller." Though it'll probably spark another novel....