The Book Thief, a novel by Australian author Markus Zusak set in Germany during WWII, was published by Knopf Books for Young Readers to much critical acclaim in March 2006. By early 2007 it had appeared on many Best Books lists and won a Printz Honor. But what has been truly unusual about The Book Thief is that its sales—to adults as well as young readers—have risen steadily since publication. The book is a regular fixture on bestseller lists more than four years later, and it has sold more than 1.5 million copies across print, audio, and e-book formats in North America.

It's a success story that not even Zusak imagined. "I honestly thought it would be my least successful book," he recently told the Palm Beach Post. "I thought, ‘A book set in Nazi Germany, narrated by Death, and it's 580 pages long. Who wants to read that?' It shows that I know nothing about publishing and what readers want to read, because out of the five books I’ve written, this is by far the most successful.”

The folks at Random House have been surprised by the book's trajectory, too, though they were always certain that The Book Thief was something special. "We had published Markus's remarkable I Am the Messenger the year before, and it did well," said Chip Gibson, president and publisher of Random House Children's Books. "So we were delighted to have another book from this extraordinary Australian writer."

A 10,000–15,000 first printing was initially planned, but that quickly changed. Gibson recalled the moment he knew that this project was unusual. "I was on a flight with our sales director, Joan DeMayo, who loved to load up with manuscripts before a trip and then shed pages along the way. She brought The Book Thief—and the manuscript was quite fat; carrying it around was a commitment! By the time we landed, Joan could not stop reading. She said, ‘We have something amazing,' and insisted that I take it immediately. I had the same reaction. The whole company fell headlong in love with the book and all our expectations were recalibrated."

In Zusak's native Australia The Book Thief was marketed as an adult book. Erin Clarke, senior editor at Knopf BFYR, had acquired both I Am the Messenger and The Book Thief as YA titles, and Gibson believed there was no reason to change that. The strategy was to get readers to read it, regardless of age. Gibson felt strongly that The Book Thief was a title that begged to be talked about; one promotional effort included offering free books to book clubs. "It's a book about books, and about the power and resonance of the written word," he said. "It's the kind of book you inhabit, and then you want to tell people where you've been living."

The book club push helped get the ball rolling, but booksellers were in the mix early as well, singing The Book Thief’s praises. "I still have the reader's copy," said Ann Seaton, manager of Hicklebee's in San Jose, Calif. Hicklebee's voted the title its Book of the Year for 2006. "It's a huge favorite, still," said Seaton. "I don't know of anything else that covers all aspects of war from all different angles."

At University Book Store in Seattle, The Book Thief nabbed that store's 2007 UBIE Award for the favorite novel of the year, according to children's book buyer Lauren Mayer. "Everyone on staff has such a passion for The Book Thief,” she said. “It’s one of our favorites to handsell to kids and teens as well as to parents and teachers. We're pretty upfront about the subject matter, letting people know that there are difficult issues, but the book is so beautifully written and so moving that it deserves to be read."

For Becky Anderson, owner of Anderson Bookshops in Naperville, Ill., "The Book Thief has been that perfect read—one that crosses over both ways—from adult to young adult and back. New readers to this phenomenal title are coming in all the time, brought in by a never-ending stream of word-of-mouth raves. "

The Book Thief continues to reach new audiences via such platforms as Community Read programs and the Jewish Community Center festival circuit. This past spring, The Book Thief was chosen as the One Book, One Community title for Read Together Palm Beach County. Darlene Kostrub, executive director of the county's Literacy Coalition, said the selection of The Book Thief, though it exceeded the group's usual 400-page limit, proved a rousing success. "What really gave Markus a huge fan club here in Palm Beach was his appearance via Skype in May," she said. "We had 500–600 people at the event and Markus was so charming and self-effacing. He answered a number of questions from the audience and it was a wonderful community event."

And so it goes. Gibson at Random House said that The Book Thief has transcended any normal velocity curves. "Sales are pretty steady. We might see a surge for graduation or back to school or Christmas. It ebbs and flows, but never ebbs too far." And Gibson is pleased to go with that flow as long as possible. "There's nothing better or more fun than to start with excitement in-house about a manuscript that's not out of nowhere—but almost—and share it and sell it."

Zusak's next book, a coming-of-age novel called Bridge of Clay, is scheduled for 2011, also from Knopf. And in The Book Thief's wake, Gibson noted, "A lot of people are eager to read that."