The book, titled Grey, is a reworking of the original book which was a worldwide bestseller and inspired a hit film and a surge of erotic novels.
Jamie Dornan played Christian Grey and Dakota Johnson was his lover Anastasia Steele in the film version of the story of a sadomasochistic affair.
James said: “Christian is a complex character and readers have always been fascinated by his desires and motivations, and his troubled past. Also, as anyone who has ever been in a relationship knows, there are two sides to every story.
“It’s been a great pleasure to return to my happy place — writing, being with Christian and Ana in their universe, and working with the fantastic publishing teams in the US and the UK.”
The book will come out on June 18, the fictional Christian’s birthday.
James said she wrote the book after huge demand from fans. On the opening page, she says it is “dedicated to those readers who asked ... and asked ... and asked ... and asked for this”.
The trilogy — Fifty Shades Of Grey, Fifty Shades Darker and Fifty Shades Freed — has sold more than 125mcopies around the world.
It has made James a millionaire many times over. This year’s Sunday Times Rich List estimated her fortune at £75m (€104m).