Vintage Books, an imprint of publisher Knopf Doubleday, said that the erotic trilogy Fifty Shades has hit 10 million in sales, just six weeks after arriving in stores, The Hollywood Reporter said.

Knopf has had to go back to press for 60 printings to meet demand. Meanwhile, Universal and Focus Features won a multimillion-dollar bidding war for movie rights.

Amazon announced this month that Fifty Shades of Grey, the first book in the trilogy by E.L. James, had supplanted The Hunger Games as its best-selling book of 2012. The three books in the series also top The New York Times, USA Today and Publisher’s Weekly best-seller lists.

“This is an astonishing number,” said Anthony Chirico, president of Knopf Doubleday Publishing Group. “The sales velocity for Fifty Shades of Grey is unprecedented, with reader demand still growing. BookScan data indicates that the trilogy has captured 25 percent of the adult fiction market in recent weeks. As we head into the heart of the summer reading season, sales are likely to continue exceeding even our most optimistic forecasts.”

The novels tell the story of Anastasia, a college student (and virgin) who gets involved in a romantic relationship with the slightly older billionaire Christian Grey that involves S&M and bondage.

James originally wrote the novels as Twilight fan fiction that she posted on the web, before reworking stories with original characters. A small Australian publishing house issued ebook editions, which generated healthy but not unusually large sales.

Vintage Books acquired English-language publication and audio rights to the Fifty Shades trilogy in February. The Vintage eBook editions were published in March, the Vintage trade paperback edition of Fifty Shades of Grey was published April 3, and the Vintage trade paperback editions of Fifty Shades Darker and Fifty Shades Freed were published April 17. In the weeks following the Vintage publications, rights to the novels have been sold in 37 countries.

The New York Times set off an explosion of interest when it dubbed the series "mommy porn" and cited its popularity among trend-setting new mothers in the city.

An intense bidding war for the movie rights ensued. Universal and Focus Features reportedly paid $5 million for the rights, despite demands for script and casting approval from James.