SJ Bennett has a short cozy series featuring Queen Elizabeth II as the detective. The fifth and most recent (2025) book in the series, The Queen Who Came in from the Cold, is on sale at Kindle UK right now for £0.99. Sadly not on sale at Kobo UK, but probably price-matchable…
Buy at: Kindle UK | Kobo UK (for price check/match)
And if you like the idea of a queen as a detective, Karen Harper has a historical series featuring the original Queen Elizabeth as the detective. Info is here, on the SYKM webpage.