J.K. Rowling is best known for penning the Harry Potter series. She has sold more more than 450 million copies, which have been translated into 73 languages and produced eight blockbuster movies, making her the first billionaire author.

But now, Rowling is moving away from the fantasy children world, releasing a new novel for adults, The Casual Vacancy.

The Casual Vacancy is set in a small community, which involves writing characters who are adolescents all the way up to people in their sixties.

"I desperately needed to write this book," Rowling told "Nightline" co-anchor Cynthia McFadden. "Not because I wanted to prove anything, not because I felt I had to write a novel for adults. None of that. You know, people ask me that sort of question a lot. I've never sat down to write anything, thinking, 'What am I proving today?' Never. It's just not how I would write.... That was just a story I really wanted to tell."

Rowling's new work is getting mixed reviews from all over the globe.

The Guardian calls the book "solid, traditional and determinedly unadventurous English novel... a study of provincial life."

"The book has a righteous social message, about responsibility for others, and a great big plot that runs like clockwork," the reviewer wrote. "On the other hand, the novel is very much a prisoner of its conventions. The plot is often predictable; it requires a large helping of artificial contrivance; and it lurches into melodrama in the final act."

The New York Times said the book is "willfully banal, "depressingly cliched," "disappointing" and "dull."

"This novel for adults is filled with a variety of people like Harry's aunt and uncle, Petunia and Vernon Dursley: self-absorbed, small-minded, snobbish and judgmental folks, whose stories neither engage nor transport us," the reviewer wrote.

According to the Daily Mirror, the book is "adult stuff that requires a very broad mind as well as patience."

"The Causal Vacancy" is already a bestseller online - No. 1 on Amazon's top 100 bestselling book list, after being featured for 82 weeks.

But Rowling said she knows that her new work won't sell as many books as Harry Potter because "lightning doesn't strike twice."

"But I accepted that a long time ago, I think back in 2000," Rowling said. "I remember thinking this won't happen again. It will never happen again... I'm not complaining about that, but 'Harry' is done."