Prince Harry isn't holding back in his new memoir Spare.

The unprecedented book, which will be released on Jan. 10, gives a candid look at the Duke of Sussex's life in the royal spotlight.

In addition to discussing his mother Princess Diana's death and how it affected him growing up, he also details his strained relationship with various members of the royal family, including his brother Prince William.

Published by Penguin Random House, the memoir was first announced in July 2021.

"I'm writing this not as the prince I was born but as the man I have become," Prince Harry said in a statement at the time. "I've worn many hats over the years, both literally and figuratively, and my hope is that in telling my story — the highs and lows, the mistakes, the lessons learned — I can help show that no matter where we come from, we have more in common than we think."

He continued, "I'm deeply grateful for the opportunity to share what I've learned over the course of my life so far and excited for people to read a firsthand account of my life that's accurate and wholly truthful."

In October 2022, Penguin Random House revealed the book's title would be Spare, a nod to the "heir and the spare" adage, pointing to different destinies that have separated Harry and his older brother William since birth.

Read ahead for the biggest revelations from Harry's book so far.

Prince Harry dubs his brother Prince William his 'archnemesis'

In a preview from Harry's Good Morning America interview, host Michael Strahan inquires about his choice of wording for his brother William, noting how he calls him his "beloved brother" and his "archnemesis" in the book.

"Strong words. What did you mean by that?" Strahan asked, to which Harry replies, "There has always been this competition between us, weirdly."

"I think it really plays into, or was played by, the 'heir/spare,' " he continued, referring to William's role as the future king while he has moved down the line of succession as William and Kate had children.

Prince Harry says that King Charles III declared him a "spare" on the day he was born

According to a leaked extract reported by The Guardian, Harry claims that his father King Charles III thanked Princess Diana for giving him a spare after his birth in September 1984.

"Early on, Harry recounts the story of how his father, now King Charles, supposedly said to his wife, Princess Diana, on the day of Harry's birth: 'Wonderful! Now you've given me an heir and a spare – my work is done,' " The Guardian reported.

The U.K. outlet adds that Harry's "resentment of being the 'spare' " is a "unifying theme" of the book.

Prince Harry drove through the tunnel where his mother Princess Diana died at the same speed

In PEOPLE's exclusive excerpt from Harry's memoir, he recounts the emotional moment he drove through the same Paris tunnel where his mother died in 1997, purposely asking the driver to drive at the same speed she was going when her and boyfriend Dodi Fayed's car crashed.

"It had been a very bad idea," he recalls in the book. "I'd had plenty of bad ideas in my twenty-three years, but this one was uniquely ill-conceived. I'd told myself that I wanted closure, but I didn't really. Deep down, I'd hoped to feel in that tunnel what I'd felt when JLP gave me the police files—disbelief. Doubt. Instead, that was the night all doubt fell away."

He continued, "I go the closure I was pretending to seek. I got it in spades. And now I'd never be able to get rid of it."

Prince Harry claims Prince William and Kate Middleton encouraged him to wear his Nazi costume in 2005

In his memoir, Harry reflects on the controversial Nazi costume he wore in 2005, claiming that it was actually William and Kate who encouraged him to wear it, as reported by Page Six.

According to the outlet, Harry writes that the Prince and Princess of Wales "howled with laughter" when they saw him in the outfit in 2005. The outlet also reports that Harry claims he debated between dressing as a Nazi or a pilot for the "Native and Colonial"-themed costume party, and asked William and Kate for advice.

"I phoned Willy and Kate, asked what they thought. Nazi uniform, they said," Harry writes, saying he brought the costume home and tried it on. "They both howled. Worse than Willy's leotard outfit! Way more ridiculous! Which, again, was the point."

Prince Harry felt he lost his brother 'forever' after his wedding to Kate Middleton

According to an excerpt of his memoir published by Us Weekly, Harry recalls that William's wedding to Kate felt like "yet another farewell."

"The brother I'd escorted into Westminster Abbey that morning was gone — forever," Prince Harry reportedly wrote. "Who could deny it? He'd never again be first and foremost Willy. We'd never again ride together across the Lesotho countryside with capes blowing behind us. We'd never again share a horsey-smelling cottage while learning to fly. Who shall separate us? Life, that's who."

Prince Harry added, "And I recall Willy walking her back up the aisle, and as they disappeared through the door, into the carriage that would convey them to Buckingham Palace, into the eternal partnership they'd pledged, I recall thinking: Goodbye."

Meghan Markle's comment about Kate Middleton's 'baby brain' caused a heated exchange between the duchesses

Per the Daily Mail, Prince Harry describes a confrontation between Kate Middleton and Meghan Markle after his wife referenced Kate's "baby brain because of her hormones" during a discussion ahead of Prince Harry and Meghan's May 2018 wedding.

Kate, who gave birth to her third child Prince Louis one month before the Duke and Duchess of Sussex's nuptials, decided to raise the prior conversation with Meghan.

"It caused such a huge row because Meghan was told that she wasn't close enough to her sister-in-law to discuss her hormones, and it wasn't the way people spoke to each other within the Royal Family," a source told the outlet. Meghan, in turn, was reportedly offended by the reprimand.

Prince Harry claims Prince William attacked him during a heated argument over Meghan Markle

According to a leaked extract reported by The Guardian, Harry details a specific argument with his brother in 2019, which reportedly turned physically violent.

Harry claims the argument started when William called Meghan "difficult," "rude" and "abrasive," to which Harry replied that his brother was simply "parrot[ing of] the press narrative" about his wife.

The Guardian reports that Harry then accused his brother of acting like an heir, after which insults were exchanged. William reportedly claimed he was just trying to help.

"Are you serious?" Harry said in reply, per the report. "Help me? Sorry – is that what you call this? Helping me?"

Harry claims that he tried to diffuse the situation by giving William a glass of water before the situation escalated.

"He set down the water, called me another name, then came at me. It all happened so fast. So very fast," Harry writes in Spare, as reported by The Guardian.

"He grabbed me by the collar, ripping my necklace, and he knocked me to the floor. I landed on the dog's bowl, which cracked under my back, the pieces cutting into me. I lay there for a moment, dazed, then got to my feet and told him to get out."