Roy Grace creator Peter James recently spent the day working as a concierge at the iconic Grand Hotel, Brighton. But having sold more than 23 million books, it wasn’t because he needed the cash. That said, he chuckles when he tells me: “I did get a fiver tip for carrying an elderly couple’s luggage out!”

In fact, the prolific author was researching the as-yet-unfinished 22nd novel featuring his Brighton-based cop, much of whose action takes place in, you guessed it, a major (fictional) hotel on the seafront of the bohemian East Sussex city. It’s a very Peter James thing to do. Old school and exhaustive in his research, he famously seeks out people, and experiences, to help him make Grace’s exploits as head of major crime for Sussex Police as thrilling – and realistic – as possible.

No wonder the series is such a hit, celebrating its 20th anniversary this year with an acclaimed ITV adaptation, starring John Simm as the titular DS Grace and Ritchie Campbell as Detective Glenn Branson, having just wrapped filming of its sixth season. Which brings us neatly to his current bestseller, his 21st Grace book, The Hawk Is Dead, and the extraordinary, “pinch myself” circumstances surrounding its writing.

For among the characters populating Peter’s fictional world are two rather famous real-life personages – namely King Charles and Queen Camilla. And much of the drama takes place in, you guessed it again, Buckingham Palace. Quite frankly, it’s a brilliant wheeze and the plot – featuring the Royal Train, an assassination bid, art thieves and a dramatic chase through the palace – is as drum-tight and compelling as ever. But how on earth did he get the royal seal of approval?

After all, as he admits with a smile: “I’m absolutely certain no other author has written a novel featuring King Charles and Queen Camilla and shown the inner workings of Buckingham Palace and the Royal Household as I’ve done.”

In fairness, it’s been known that Camilla, 78, was a Roy Grace fan since lockdown, when the then Duchess of Cornwall was photographed working from her delightfully cluttered study at Birkhall in Aberdeenshire.

Among mementoes, photographs and letters, it was Camilla’s “shelfie” – the image of her packed bookshelves – that attracted the most attention, including as it did several of Peter’s distinctive neon-spined thrillers. She subsequently revealed that Roy Grace is her favourite fictional detective.

“I wrote a thank you note and got back a handwritten two-page letter telling me how much she loved my books,” says Peter. “I would then send her each new book and she would always write back a very chatty letter. Then shortly after becoming Queen, she wrote asking if I might set a Roy Grace novel in London. Well, things have not ended well for people who’ve disobeyed past Queens of England… so I started thinking about it.

“Then a senior member of the Royal Household told me Her Majesty was serious – she would love to see Roy Grace in London and perhaps Buckingham Palace might be a good location? Perhaps there might be a murder in the Palace… or two…? Might I come up with an idea he could run by her?”

By then the creative cogs were turning. Peter’s biggest problem, he explains, was how to get a detective based in Sussex investigating a death in London. He came up with the perfect solution: a murder on board the Royal Train.

“I thought, ‘How about Queen Camilla coming by train to Brighton for the start of a two-day tour of hospices along the south coast?’ Roy has been tipped off that there’s a planned ‘Not My King’ protest at Brighton Station to greet the Queen,” he explains.

“I’ve always been fascinated by the Royal Train which has been used by members of the Royal Family as a kind of ‘hotel on wheels’. Rather than return to London each night, it’s parked in a siding and guarded. Its nine carriages contain bedrooms, an avocado bathroom suite for the Royals, a lounge and office.”

But in his new thriller, as it chunters through Clayton Tunnel, five miles north of Brighton, the train suddenly derails. Having helped her shaken entourage off the train, Camilla and her party stumble along the tracks and into daylight – where a senior member of the royal household is shot dead by a sniper in some of the most graphic writing featuring ‘the Firm’ since Prince Harry’s memoir, Spare.

“The top half of Sir Peregrine Greaves’ head literally exploded in a pink cloud.,” writes Peter, 77, in The Hawk Is Dead. “The Queen’s clothing was splattered with blood and pale brown particles.” It’s certainly not for the faint-hearted.

“Everyone immediately assumes it’s a failed assassination attempt on the Queen,” says the author. “Everyone except DS Roy Grace who is quickly on the scene as this is his turf.”

Grace maintains that a sniper might miss by four inches, but not four feet – and the dead man, Sir Peregrine, was standing four feet from Her Majesty. Peter continues: “As you can imagine, this immediately brings him into conflict with everyone, and he’s summoned to Buckingham Palace by a desperately worried and irate King Charles to explain how he can possibly believe this was not an attempt on his wife’s life.”

The ensuing investigation will take Grace and his team deep into a conspiracy at the very heart of Buckingham Palace. It’s thrilling stuff, and also a fun tribute to the Royal Train which, sadly, is due for retirement by March 2027.

“I sketched out a very brief outline and sent it to Her Majesty via a member of the Royal Household. He liked it and said he would run it by ‘HMQ as he called her,” says Peter. “A week later, he emailed to say she loved it. He just needed to get it signed off by Buckingham Palace comms. Two days later he told me I was good to go.”

It was really that easy? Yes, chuckles Peter, and to put the icing on top of the cake, the author was allowed an exclusive tour of the Palace.

“To help me with my research, I was given a three-and-a-half hour tour behind the scenes at Buckingham Palace followed by much enthusiastic help from several members of the Royal Household at all levels,” he says. “The Queen herself was brilliant, also.

“Writing The Hawk Is Dead was an incredible journey, and one in which I sometimes had to pinch myself – I walked around the basement, the corridors and the rooftop of Buckingham Palace. With every Roy Grace novel I try to write a very different book – but this is definitely the most different of them all.”

Has Camilla read the finished book, I wonder?

“I was very concerned, as although I fictionalised all members of the Royal Household, I felt the book would not feel authentic if I gave King Charles and Queen Camilla fictional names,” he continues. “My publishers bound a very early uncorrected proof in March, printed 001 in gold on the cover, and I had it hand-delivered to the Queen, with a note telling her it was the only copy in existence and if there was anything she wanted changed, now was the time.

“I was invited to Clarence House the following week. I didn’t sleep a wink the night before, worrying she was going to tell me she hated it and that I could not publish it. But, to my amazement, she came over to me with a big smile and said, ‘I absolutely love it!’ I asked her if there was anything she wanted changed and she said, ‘Not one word’.”

As for the King, Peter, who has homes in Worthing, West Sussex, and Jersey and is married to second wife Lara James, continues: “I’m told the King rarely reads fiction, but I have heard that the Queen has shared bits of the story with him and he thinks it is great. I also met him last July in Jersey.

“He was utterly charming, shook my hand and said, ‘I want to thank you so much for how you keep my wife so entertained. How on earth do you keep coming up with your storylines?’”

It’s a good question. “For me the absolute key is, whilst keeping my principal characters – and just occasionally killing one off, to write about things that both interest me and that I want to learn about,” Peter adds.

It’s somehow fitting that the Queen, who has done a lot to try and keep people reading, should be immortalised in print in this way. Having started a book club on Instagram during lockdown, The Queen’s Reading Room was officially launched as a charity two years ago with the aim of encouraging reading around the world. Its debut festival took place in June 2023 at Hampton Court Palace. Since then, the charity has funded research into the benefits of reading.

“The initial findings are that just reading five minutes a day has massive benefits for both mental and physical health,” says Peter. “She has followers in almost every country in the world and is, in my view, the greatest champion of reading of any member of the Royal Family, ever.”

With season seven of Grace going into production next year for ITV, perhaps Peter can persuade the Royal couple to appear as themselves? “Well, King Charles loved to act when he was a teenager, and was very adept at some of the Goons’ impressions,” he chuckles. “So maybe they’d like to take a few weeks out from Royal duties and play their roles!”

• The Hawk is Dead by Peter James (Macmillan, £22) is out now