J.K. Rowling, the British author whose “Harry Potter” fantasy series ignited a passion for reading for millions of children around the world, has emerged from a five-year publishing hiatus with a new book: this time for adults. Little, Brown and Company, part of the Hachette Book Group, said on Thursday it had acquired the rights to publish the book, whose title and publication date was not named.

“Although I’ve enjoyed writing it every bit as much, my next book will be very different to the Harry Potter series, which has been published so brilliantly by Bloomsbury and my other publishers around the world,” Ms. Rowling said in a statement. “The freedom to explore new territory is a gift that Harry’s success has brought me, and with that new territory it seemed a logical progression to have a new publisher. I am delighted to have a second publishing home in Little, Brown, and a publishing team that will be a great partner in this new phase of my writing life.”

The first Harry Potter book was released by Scholastic in the United States in 1998 under the name “Harry Potter and the Sorcerer’s Stone.” Six more novels in the series followed, quickly becoming the fastest-selling books in history. They have been translated into roughly 70 languages. In 2007, on the first day of publication of “Harry Potter and the Deathly Hallows,” the seventh and final book in the series, the book sold 11 million copies. (Ms. Rowling also released “Tales of Beedle the Bard,” a collection of fairy tales whose net proceeds went to a charity Ms. Rowling co-founded.)

The series was an enormous boon to bookstores, which sponsored midnight parties when the books were released, drawing both children and adults who dressed in full costume.

Ms. Rowling and the Harry Potter series made news last year with the announcement of Pottermore, an interactive Web site where e-book editions of the Harry Potter books would be available for the first time, as well as fantasy gaming components that would be tied into the books. The site was scheduled to be introduced last October, but has been delayed.