What sets the book apart is that it is written for women by a woman - British author EL James, a previously unknown TV executive.
"Never before has a book aimed at an adult audience sold so many copies so quickly," the book's British publisher Random House says.
Only the seven-volume Harry Potter series can rival that sort of success - although the subject matter is vastly different, of course.
James - a 48-year-old mother of two whose real name is Erika Leonard - has invented a new genre dubbed "mommy porn", a hybrid of cheesy romance and erotic fiction.
Her hero Christian Grey is a mysterious millionaire, with "nothing to envy of Michelangelo's David", who has a steamy entanglement with Anastasia Steele.
Shortly after their first encounter, there is the first mention of a spanking and there are liberal doses of whips and handcuffs for sado-masochistic routines.
The trilogy runs to 1500 pages and takes the reader on a discovery of millionaire Grey's tormented soul - with plenty of sex and dark, bondage-driven scenes thrown in.
James, who says she was inspired by the teenage vampire film Twilight, started by publishing extracts on a fansite for the American teenagers' series before releasing an e-book in May 2011.
It was an instant hit, with hundreds of thousands of downloads.
"We were offered Fifty Shades on March 6, we met with the author and her agent on March 8 and had bought the trilogy by March 12," Susan Sandon, managing director of Random House, told AFP.
"A few of the senior team read it overnight and knew that this had all the hallmarks of a bestseller. It's a Mills and Boon Love story with plenty of sex."
Since its release in paper form it has been a huge hit - 31 million copies have been sold in English, and 20 million copies in the United States, where some bookshops initially refused to sell it because of its racy content.
"It's still selling half a million copies a week in Britain," said Jon Howells, of bookseller Waterstone's.
A total of 43 countries have bought up the rights, the author's agent Valerie Hoskins told AFP.
Five million copies have already been translated into languages other than English.
Yet behind the hype, literary critics are not impressed.
Fifty Shades suffers from "improbable" dialogue, "lame characterisation" and "irritating tics", The Guardian newspaper said, echoing the majority of critics.
But most readers aren't expecting a masterpiece and are just pleased to be able to buy such a book in the high street, rather than in the darkened corners of a sex shop.
"I haven't stopped reading it," confides Janine, a young British reader.
"Though it's badly written it really surprised me how I couldn't put it down!" said another.
"It's a Mills and Boon Love story with plenty of sex," another reader said, one of around 4500 who have posted their views on online seller Amazon's UK site.
"It's causing so much interest because we are actually willing to face our desires now," said Michelle Bassam, a sex therapist based in London.
"A lot of women look at porn to actually get ideas of certain behaviour.
"Previously that was never going to be an option for women - or so we were led to believe."
But the publisher and bookshops say that men are just as interested in reading it.
"What has surprised us is to see that people who normally never buy books are rushing to buy it", Sandon said.
As for James, she is already working on her next project at her west London home, but it remains a closely guarded secret.
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