The book by celebrity biographer Sean Smith, called Meghan Misunderstood, is set to be released in November. It promises to “set the record straight on the most talked about, unfairly vilified and misrepresented woman in the world”.
Promotional material for the book says Meghan had an “extraordinarily accomplished life” before she met Prince Harry.
It describes their royal wedding as a “joyful occasion”, and adds: “Theirs was a story that the screenwriters of Hollywood – where Meghan had made her name – could scarcely have imagined.”
But it says the “rom-com fantasy” turned into a “disturbing drama”.
The promotional material adds that the author “pulls no punches as he reveals the remarkable and powerful story of this self-made, intelligent American woman with a strong social conscience who has made such an impact on our lives”.
The Sussexes had hoped to continue supporting the Queen while earning their own money but the idea was deemed unworkable amid fears they would be accused of cashing in on their royal status.
Meghan, 38, and Harry, 35, are now living in the US after quitting royal duties on March 31.
A joint biography of the Sussexes, titled Finding Freedom, is also being published in August.
Authors Omid Scobie and Carolyn Durand previously said they wanted to tell the “definitive story” about Meghan and Harry.
They said: “The aim of this book is to portray the real Harry and Meghan, a couple who continue to inspire many around the world through their humanitarian and charitable work but are often inaccurately portrayed.
“Our mission has been motivated by a desire to tell an accurate version of their journey and finally present the truth of misreported stories that have become gospel simply because of the number of times they have been repeated.
“It is thanks to our sources that we have been able to share the definitive story of the Duke and Duchess of Sussex.”