The Windsor knot / S. J. Bennett.

Windsor Castle, 2016. The morning after her ninetieth birthday celebrations, the Queen is shocked to discover that one of her guests has been found dead in his room. It quickly becomes clear that his death was no accident, but when MI5 begin to suspect her loyal servants, she knows that they are looking in the wrong place. For the Queen has been living a double life ever since she was a teenager: by day she upholds her royal duties, while secretly solving cases as an amateur detective. With the Crown's reputation on the line, her investigations must be kept under wraps - and she must always ensure that someone else gets the credit. With the help of her Assistant Private Secretary Rozie Oshodi, it's up to the Queen to find the real killer - before they strike again.