Fifty Shades Of Grey has become the first book to sell more than one million copies on the Kindle.
Online retailer Amazon.co.uk said British author EL James had already become its best-selling author of the year.
And it said the novel - the first in James' Fifty Shades trilogy - was the biggest Kindle ebook yet.
James - real name Erika Leonard - has had colossal sales for her erotic fiction books, with the paperback version of her debut achieving the UK's highest weekly sale for a paperback.
Fifty Shades Of Grey, which is mainly set in Seattle, traces the deepening relationship between college graduate Anastasia Steele and young businessman Christian Grey.
The trilogy is notable for its explicitly erotic scenes which include elements of bondage and sado-masochism.
Amazon said the Kindle edition of Fifty Shades Of Grey was outselling the print book at a rate of more than two to one.
James' trilogy of books was published in March, and the film rights have already been snapped up.
According to Deadline New York, Angelina Jolie is keen to direct following the success of In The Land Of Blood And Honey, her debut as a film-maker.
Bret Easton Ellis, who wrote American Psycho, has already expressed his desire to write the screenplay for the film.
He tweeted: "I'm putting myself out there to write the movie adaptation."
The print versions of James' books - the others are Fifty Shades Darker and Fifty Shades Freed - take the top three places in Amazon's bestseller lists.
Last week the paperback of Fifty Shades sold more than 200,000 copies, establishing a new weekly record.
James' husband Niall Leonard has also landed a publishing deal for his debut novel, a thriller called Crusher aimed at young adults.