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A Death in Diamonds by S. J. Bennett
I've read this series from the beginning and thoroughly enjoy seeing Queen Elizabeth II as an amateur sleuth. She knows people well and has access to a lot of information but needs her assistant private secretary to do most of the legwork. This time around we've gone back in time to the late 50s. It's early in the Queen's reign and she has two young children. She and Prince Phillip have been married about 10 years and their marriage has more stressors than most, one being that a club Prince Phillip attends has been connected to a murder.
Joan, the assistant private secretary, is bold, resourceful, and intelligent. She is the only one the Queen can truly trust, since in addition to the murder case, they are trying to figure out which of the Queen's advisors has been attempting to sabotage her trips abroad.
The peeks into royal life are fun. The queen visits a few other countries in this one and...