* Adult book's title and plot details not released (Adds details)

By Christine Kearney

NEW YORK, Feb 23 (Reuters) - J.K. Rowling, whose "Harry Potter" fantasy book series became a global publishing phenomenon, is writing her first novel for adults but the title and plot are still a closely-guarded secret.

The British writer, 46, whose teenage boy wizard tales became international best-sellers and inspired a series of hit films, said on Thursday that her new novel would be "very different" to the "Harry Potter" books that made her a household name and turned her into a billionaire.

"Although I've enjoyed writing it every bit as much, my next book will be very different to the Harry Potter series," 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."

Publisher Little, Brown and Company, part of the Hachette Book Group, acquired the rights to publish the book, whose title and publication date will be announced later this year. It is Rowling's first major novel in several years after the last "Harry Potter" book in the series was published in 2007 and a supplement book in 2008.

Rowling's "Harry Potter" series was published by Scholastic in the United States, Bloomsbury in Britain and other publishers around the world.

"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."

Neither Rowling nor a spokeswoman for Little, Brown and Company released the terms of the deal or whether any more adult novels were planned. The books will be released in print and e-book form.

Such is the scale of Rowling's fan base and the success of her novels that the news of an adult novel is a boon for the book publishing world, which has struggled in recent years as it crosses over into e-books.

The influential 46-year-old writer's seven-book "Harry Potter" saga between 1997 and 2007 sold 450 million copies worldwide and the eight movies from the Warner Bros. studios have taken more than $7.7 billion at global box-offices, making the films the largest-grossing franchise in history.

The books about a teenage wizard and his friends battling good and evil in a world of witchcraft also spawned a host of fan clubs around the world dubbed Pottermania, and even theme parks, some still in the making.

Last year Rowling unveiled Pottermore, a website allowing fans to interact with the characters and storylines and the books were due to be released in e-book form for the first time, but the website has been delayed.

After Rowling announced that website last year, the author said she had "closure with Harry" and had no plans to write another Potter novel.

But while many adult fans emerged from her boy wizard "Harry Potter" series, it is yet to be seen if Rowling can achieve the same success with a novel directly aimed at adults. (Editing by Mike Collett-White and Jill Serjeant)

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