NEW YORK (REUTERS) - E.L. James, the British author of the hugely successful Fifty Shades Of Grey erotic novels, is celebrating the fictional birthday of her famous male character on Thursday with the release of her new book, Grey, which looks set to be an instant bestseller.

Publisher Vintage Books said demand for the novel, told from the viewpoint of business tycoon Christian Grey, has been consistent with the other books in the trilogy, which have sold more than 125 million copies worldwide.

Vintage, a division of Knopf Doubleday Publishing Group, plans to have one million copies ready for sale in bookstores on Thursday.

Grey has been the top Kindle pre-order book of 2015 on Amazon, and sales are expected to soar when the e-book is released at midnight on Wednesday.

The book will also be published in Britain and in audio and Spanish-language versions.

"It is the highest pre-order of the year," said Sara Nelson, editorial director at Amazon.com.

"I would be very surprised if it is not a bestseller."

Fifty Shades Of Grey is the all-time best-selling Kindle title.

James announced the surprise launch of the book earlier this month in one of the best-kept secrets of publishing.

She was prompted to write it by fans who had asked her to tell Grey's side of the story of his erotic relationship with virginal college student Anastasia Steele.

"Christian is a complex character," James said when she announced the book's release date on the Instagram social media network, "and readers have always been fascinated by his desires and motivations."

Grey is a handsome, 27-year-old self-made billionaire with a troubled past who lives and works in an ordered world that becomes unsettled when he meets Steele and introduces her to kinky sex, sadomasochism and bondage.

In Grey, James will give her millions of avid fans Grey's take on his relationship with Steele and his thoughts and insights.

"Books like this always have an appeal," said Amazon.com's Nelson.

The first book of the trilogy was turned into an R-rated film by Comcast Corp-owned Universal Pictures starring Dakota Johnson and Jamie Dornan, grossing more than US$500 million (S$670 million) at the global box office since its February release.

Several countries, including India, Malaysia, Indonesia and Kenya, banned the film from their theatres because of its erotic content.

Critics said the movie blurred the lines between healthy and harmful sex.

Film adaptations of the second and third books are also planned.