It does not bode well for King Charles. The Duke of Sussex’s ghostwriter, JR Moehringer, is an expert in fraught father-son relationships.

His 2005 memoir, The Tender Bar, is the story of an absent father, who was an “unstable mix of charm and rage” and left Moehringer in need of “men as mentors, heroes [and] role models”.

His best-known and successful work, as ghostwriter for Andre Agassi’s autobiography Open, revealed how domineering and tunnel-visioned the sportsman’s father was. Emmanuel Agassi hung a tennis ball mobile above his infant son’s crib and taped a ping-pong paddle to his hand to encourage him to bat them away.

It is no wonder the palace is said to be particularly concerned about the revelations in Prince Harry’s memoir, Spare, about his relationship with his father.

Ghostwriters are usually one of publishing’s dirty secrets, and Moehringer’s name was left off the announcement from Penguin Random House that it was publishing the book.

But Moehringer, 57, who lives in California, is one of the most high-profile ghostwriters of his generation, and will be integral to the book’s success. He is believed to have been introduced to the couple by their friends, George and Amal Clooney. George directed last year’s film adaptation of The Tender Bar, which starred Ben Affleck as Moehringer’s Uncle Charlie, who owns the titular bar.

JR Moehringer’s 2005 memoir, The Tender Bar, is the story of an absent father, who was an “unstable mix of charm and rage.”

Publishing insiders have been impressed by the choice of ghostwriter for Spare, which will be released on January 10. “The Tender Bar is extraordinary. JR is the real deal,” said Patrick Janson-Smith, who published Moehringer’s 2012 novel Sutton when he ran Blue Door, a former HarperCollins imprint. “He showed that with Open too — that wasn’t just any old hack job, it was a good few cuts above the average sporting memoir.

“He lends a weight and a sincerity to everything he writes, which I think will play very well with the Harry memoir, where he will go deep into the emotions, especially of losing his mother so young.”

The historian Robert Lacey added: “I’m struck by the focus of his psychological analysis and by the power of his narrative imagery … And I’ve also been struck by how some of Harry’s [recent] statements about himself and his feelings have tended to sound more Moehringer than Windsor.”

Moehringer’s exploration of Harry’s sometimes rocky relationship with Charles will be scrutinized. Penny Junor, the royal biographer, said that the King had been “quite a remote figure” to his sons because he was “consumed” by his work. Harry was not the easiest teenager either. When he was 16 and regularly smoking cannabis and drinking alcohol, Harry was taken by Charles to a drugs rehabilitation clinic, Featherstone Lodge in south London, for a day visit to give him a shock. Harry also created “Club H” at Highgrove, throwing drug and drink-fueled parties in the cellar.

When Harry and Meghan decided to quit the royal family in January 2020, a summit was held at Sandringham with the Queen, Harry, Charles and William, where the tensions between the duke and his father and brother were clear. In March 2021, the couple gave their tell-all interview to Oprah Winfrey in which Harry said that Charles had stopped taking his calls, and that he felt “really let down” by his father, adding: “There is a lot of hurt that’s happened.”

Harry created “Club H” at Highgrove, throwing drug and drink-fueled parties in the cellar.

Moehringer and Harry come from different worlds, but there are some parallels in their lives. Moehringer’s parents had an unhappy marriage, and he dedicated The Tender Bar to his mother, Dorothy, who died in 2019. There are echoes of Diana: Dorothy was a beautiful woman whose most frequent facial expression was self-command, like “a young aristocrat posing for her coming-out portrait”. She “could be gentle and fragile, but would assuredly be fierce when protecting those she loved”.

Here, though, the similarities end. Moehringer grew up in poverty, with a father — the disc jockey Johnny Michaels — who shirked his financial obligations and whose “one true genius was disappearing”. Michaels was violent repeatedly towards Dorothy, until she was forced to leave when her son was only seven months old. As a young child, Moehringer never saw his father, so only knew him from his voice on the radio, which “was so deep, so ominous, it made my ribs vibrate and the utensils tremble”.

His mother, who had been denied a college education by her own father, wanted Moehringer to study at Harvard or Yale and become a lawyer, so he could sue his father for child support.

Moehringer, who is married to the book editor Shannon Welch, with whom he has two children, went to Yale but instead became a writer. He started as a news assistant at The New York Times, and in 2000 at the Los Angeles Times he won the Pulitzer prize for a feature about the descendants of slaves living beside a stretch of the Alabama river.

Moehringer’s mother wanted him to study at Harvard or Yale and become a lawyer, so he could sue his father for child support.

It was the Agassi book, published in 2009, that made Moehringer’s career, though. No blushes were spared — the book reveals that Agassi’s mother once walked in on her mother-in-law breastfeeding the infant Agassi — but it is considered an exceptional piece of writing too.

“It was a game-changer,” said the managing director of a rival publishing house. “It made people realize that celebrity memoirs could be of that caliber. We then had Keith Richards’s brilliant memoir, which [the journalist] James Fox co-wrote. It showed what a ghostwriter can do — what happens when you throw a literary talent into the mix. Open was honest, beautifully written and full of surprises — on the first page Agassi reveals he hates tennis.”

The question for the royal family is how embarrassing Spare might be. Harry was said to have asked for some last-minute changes, fearing parts would look insensitive in light of the Queen’s death. Foreign publishers initially received a limited section of the book, and were locked out of the rest, an indication that some chapters are being altered.

“We know Moehringer will thoroughly plumb the negative, but I’d hope he’ll also illuminate the positive — how Diana’s brother and sisters parented both boys after her death, perhaps,” said Lacey. “I’d also hope to discover more about the personal trust and intimacy that Harry clearly established with his grandmother — which took some by surprise during the unrolling of Megxit. Then there are the resentments Harry must have developed towards both his father and his brother in those years, but is not known to have voiced until the arrival of Meghan. He was such a happy and loyal ‘spare’ until she arrived and got him thinking.”

Spare, by Prince Harry, the Duke of Sussex, will be published on January 10 from Random House

Rosamund Urwin is the media editor for The Sunday Times