Locked Room Mystery
Mac is calmly playing mini golf when the queen of England phones him yet again for help. She is afraid that someone will steal the crown jewels, so wants Mac on the case. He ends up in her castle in Scotland, where all manner of things go wrong, including the jewels being taken from a locked room. That's a new and exciting type of mystery for Mac, who has to use all of his skills to solve it.
I love that the author made up all of these stories years ago and has decided to finally commit them to paper. They are very much the sort of thing a young writer would embrace, and this history gives these books a very fresh feel.
This reminded of Kin Platt's Big Max: the World's Greatest Detective (1965). Younger readers who can't get enough of Adler's Cam Jansen and Roy's ABC Mysteries will adore Mac, his corgis, and his attempts to help the queen of England solve her mysteries.