The Fifty Shades trilogy, author E.L. James' erotic saga about a young woman and the mysterious man she falls for, has sold more than 10 million copies in the United States, the publisher revealed Tuesday.

The numbers, which include audiobooks and e-books, are expected to keep going up.

As we head into the heart of the summer reading season, sales are likely to continue exceeding even our most optimistic forecasts, Knopf Doubleday Publishing Group president Anthony Chirico said in a statement.

James' story started off as Twilight fan fiction, featuring erotic scenes between author Stephenie Meyer's characters of Bella Swan and Edward Cullen. The fan fiction story was titled Master of the Universe. It was later altered and published as the Fifty Shades trilogy by an Australian publishing house. Vintage Books, which is part of Random House's Knopf Doubleday Publishing Group, acquired the U.S. rights to the series in February. The books are available in paperback, e-book and audio forms.

We have had to reprint the books on a weekly basis since publication, with some daily reprints topping 900,000 copies, Chirico said in a statement.

The books have ignited much debate about the nature of the relationship between the two main characters. Fans of the trilogy certainly love it, but some U.S. libraries have gone so far as to keep it off the shelves due to the erotic content (even though many a library romance novel contains blushworthy material).

On the flip side, the books have also prompted many a spoof. Comic actress Kristen Wiig parodied the women who have taken to the books in a Saturday Night Live skit. A video of actress Selena Gomez, titled Fifty Shades of Blue, is making the rounds. It features the cute actress of Disney fame reading the book and then falling in love with a scruffy-looking house painter.