Meg Rosoff wasn't able to celebrate the glowing reviews her first novel received when they started coming in—she had just been diagnosed with breast cancer. "I was in the hospital for my first operation when the book was released and all these flowers started arriving. Half of the cards said, 'Congratulations,' the other half said, 'We're so sorry.' "

The cancer came as no surprise—two of her sisters had already gotten it. Nor will Rosoff's relaxed humor about it startle those who have met Daisy, the 15-year-old narrator of How I Live Now (Random/Wendy Lamb), a compelling life-during-wartime story.

Daisy's self-deprecating wit and bracing honesty have captivated readers on both sides of the Atlantic. Hers is a voice Rosoff first heard in a New York friend's "really snarky" teenage daughter and which she expanded on in a notebook one day while riding on a bus.

"I just had [Daisy] introduce herself," Rosoff says, in a telephone interview from the London home she shares with her husband, Paul Hamlyn, a painter. "And it just hit me somehow—that mix of a girl who acts tough but is very vulnerable—damaged, in fact. That's what I was interested in. I wanted to take one of those horrible New York girls out of her situation and see if she could stop thinking about herself for a minute and develop as a human being."

Rosoff herself is a city girl, an ex-pat who has lived in London for 16 years but grew up in Boston. She majored in English and fine art at Harvard and worked, unhappily, in advertising for most of her professional life.

"I pretty much wanted out the minute I got in. It never suited me. I had been thinking of every possible way I could get out—knit mittens, maybe, if it came to that—but I never considered writing novels because I'm not very good at plot."

Then Rosoff's youngest sister, Debbie, was diagnosed with cancer, and died in 2001. "I spent a lot of time flying back and forth to the States, and I thought more and more, 'What the hell am I doing in advertising?' Life doesn't go on forever and you don't want to drop dead without ever having done what you wanted to do."

After her sister's death, Rosoff asked for compassionate leave and used her time off to write a "practice novel" about a girl and a horse—the sort of book she, as a devoted fan of Walter Farley and Marguerite Henry, would have read as a girl. She also penned a picture book manuscript, Meet Wild Boars, inspired by her daughter, Gloria, now seven (Henry Holt will publish the book next May, with illustrations by Sophie Blackall).

The agent who handled the picture book sale, Catherine Clarke, read the horse story and told Rosoff to write another novel. Rosoff admits, "She's very imposing. Oxford. Brilliant. I'm a little frightened of her, actually, so I wrote another book trying to impress her."

After one false start—writing in third person—How I Live Now came quickly. Rosoff had a finished manuscript in three months. "It emerged like Venus on the half-shell, and did I appreciate it at the time? No," Rosoff says. "I thought, 'Oh, writing books. This is easy. I can do four a year.' "

Clarke loved it. So did Puffin's Rebecca McNally, who published the British edition, and Wendy Lamb, who scooped up U.S. rights at auction. The book was edited simultaneously, which made Rosoff's head spin occasionally, since both editors bent over backwards to make sure the story sounded authentic on both sides of the Atlantic. "It was mostly Daisy's voice that the Americans said sometimes sounded too English, and my English editor sometimes wanted me to clarify things [Daisy] said that were too American for the U.K. audience to understand," Rosoff says. "It's because I'm so mid-Atlantic now, I don't know what's what anymore." Still, Rosoff says, revisions were minimal. The biggest change was to the ending, which she expanded.

How I Live Now has been published to much acclaim; it won the Guardian Children's Fiction Prize, and was shortlisted for the Whitbread. Film rights for the novel were acquired at manuscript stage and Rosoff has since written the screenplay. The film is being produced by Passion Pictures, with financing from the British Film Council.

Rosoff is currently at work on a second book, which needs, in her words, "a big revision." Chemotherapy makes her tired and sick, Rosoff says, but on the phone, she's upbeat—a "depressed optimist," she calls herself.

"Listen," she says. "It's better to have cancer with good reviews than to have cancer and bad reviews. It certainly has kept me grounded."