The Windsor Knot by SJ Bennett 

The morning after a dinner party at Windsor Castle, eighty-nine-year-old Queen Elizabeth is shocked to discover that one of her guests has been found murdered in his room, with a rope around his neck. When the police begin to suspect her loyal servants, Her Majesty knows they’re looking in the wrong place.

For the Queen has been living an extraordinary double life since her coronation. Away from the public eye, she has a brilliant knack for solving crimes. With her household’s happiness on the line, her secret must not get out. Can the Queen and her trusted secretary Rozie catch the killer, without getting caught themselves?