Fifty Shades of Grey is celebrating a double milestone. The erotica trilogy by EL James has sold more than 100 million copies, its American publisher, Vintage Books, said last week.
The second milestone is that the trilogy has spent 100 weeks on the New York Times bestseller list.
According to Random House, it has never had any book series sell as fast as the trilogy in its history as a publishing house.
Other series with more than 100 million copies sold include the Harry Potter and Twilight series and the Nancy Drew books, according to the New York Times.
James started writing the books as fan fiction for the Twilight series. She wanted to write a more sexualised version based very loosely on Twilight under the title of Master of the Universe, but when she put it up on fan fiction, the administrators removed it.
She then reworked her story, put it up on her own website and developed it into a trilogy involving her two main characters, Anastasia Steele and Christian Grey.
"The Fifty Shades trilogy struck a chord around the world that continues to resonate," said Anne Messitte, publisher of Vintage-Anchor Books in the US. "Two years after our initial publication, [sales] remain strong and steady."
The sales peaked at an average of two copies every second in the US, the New York Times reported.
A film adaptation of Fifty Shades, starring Jamie Dornan as Christian Grey and Dakota Johnson as Anastasia Steele, is expected to be released in February 2015.
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