'Fifty Shades' trilogy breaks 20 million sales mark in US

That makes Americans by far the biggest enthusiasts of British author E.L. James' steamy best-sellers about a 21-year-old English literature student and her sadistic billionaire lover.

"As of Monday we have sold more than 20 million copies in all formats" in the United States since its publication four months ago, Russell Perreault, director of publicity at Vintage Books, told AFP in an email.

Worldwide, the "Fifty Shades" books -- starting with "Fifty Shades of Grey" -- have sold a total of 31 million copies in all English-language markets, the Wall Street Journal reported earlier this week.

On Monday, Universal Pictures and Focus Features, who snapped up the film rights in March for $5 million, said Michael De Luca and Dana Brunetti -- collaborators on "The Social Network" -- will produce the film adaptation.

"At its core, 'Fifty Shades of Grey' is a complex love story, requiring a delicate and sophisticated hand to bring it to the big screen," said Universal's co-chair, Donna Langley, in a statement.

"Mike and Dana's credits more than exemplify what we need in creative partners, and we're glad to have them as part of our team."