Gliding into the courtyard in a chauffeurdriven car, the nature-loving royal gazed approvingly at the colourful wildflower meadow at the front of the historic palace in south-west London.
And waiting to welcome her were some of the most famous names in literature, who were ready to entertain her and the packed audience through the world of books.
HELLO! was there too, and as Her Majesty emerged in a wide-legged navy Anna Valentine jumpsuit to join a champagne reception, she mingled with literary stars including Rebus creator Sir Ian Rankin, Shetland and Vera author Ann Cleeves, historical novelist Kate Mosse and thriller writer Harlan Coben, together with classicist Dame Mary Beard and actresses Helena Bonham Carter and Patricia Hodge.
The Queen, who was accompanied by her son, food critic Tom Parker Bowles, also chatted to the King's cousin, Lady Sarah Chatto, who came with her husband Daniel and their sons, Samuel, 27, and Arthur, 25.
But it was while she was talking to Lee Child, author of the Jack Reacher thrillers, that she gave an update on a matter on everyone's minds: her husband the King's health.
MISBEHAVING HUSBANDS
In a candid conversation with the novelist, Her Majesty said the 75-year-old monarch - who was diagnosed with an undisclosed form of cancer in February and has returned to public-facing duties while undergoing treatment - is "doing fine" but "won't slow down and won't do what he's told".
The best-selling writer replied: "Sounds to me like a typical husband." Afterwards, Lee said: "I asked her how the King is because when I got my CBE, he did my investiture, so I have a soft spot for him and I said: 'I hope he's doing well."
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