Prince William and Prince Harry's ever-changing relationship will take centre stage in a new tell-all book. 

The book, by royal historian and biographer Robert Lacey, promises to get into the "untold details of William and Harry's closeness and estrangement."

Battle of Brothers: William & Harry - The Friendship and the Feuds is set for an Oct. 15, 2020 release. At that time, royal fans will get an inside and expert reading of William and Harry's bond, their struggles, and their place in royal history.

Battle of Brothers to explore "heir" and "spare" narrative

Lacey, who has previously served as a historical consultant on the acclaimed Netflix royal drama, The Crown, will reportedly take on the brothers' story from the angle of the "heir" and the "spare," reflecting on how that has influenced their presently divergent behaviours.

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Amid Harry and Meghan's decision to step down from their duties as senior royals, Harry and William's one-time strong bond reportedly grew strained. Reports have said that their relationship is "forever changed." 

The Sussexes have now been living in Los Angeles since March, and sources have said only that the brothers are "back in touch" after the tense royal exit.

Author of William & Harry book was "moved to tears" by story

According to People, Lacey described the range of emotions he encountered as he learned the details that came to form his book.

"I have been astonished and sometimes moved to tears by the fresh details and insights I have discovered in researching this story of family conflict," he said.

"It has been both enthralling and painful to trace this drama through the memories of close witnesses and some of the people most intimately involved."

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Other questions the book will address, according to its publisher, include: "How have William and Harry both agreed and diverged in their views of what a modern royal owes to their country? Were the seeds of damage sowed by Prince Charles and Princess Diana as their marriage unraveled for all the world to see?

"In the previous generation, how have Prince Charles and Prince Andrew’s own relations strained under the Crown? What role has Queen Elizabeth II played in marshalling her feuding heirs?

"What parts have Kate Middleton and Meghan Markle played in helping their husbands to choose their differing paths? And what is the real, unvarnished story behind Harry and Meghan's dramatic departure?"

Battle of Brothers comes out on Oct. 15 and is available for pre-orders now. Look for the "unparalleled insider account of tumult, secrecy and schism in the Royal family" this autumn!