This is book 14 of the series featuring Mary Russell and Sherlock Holmes. The first book was The Beekeeper’s Apprentice describing their first meeting, published in 1994. Spies came out in 2015.
In this story, the couple visits Japan and are drawn into events that endanger the life of the Crown Prince, Hirohito. The case does not resolve until they return to England, and even then the outcome is in the hands of a Japanese woman.
In case you are wondering, the Connon Doyle family does not sanction this series of stories and insist that they own the exclusive rights to Sherlock Holmes. Under US copyright law, Holmes is in the public domain and King is American.
I chose this book to read on our recent trip to Haiti and the Dominican Republic because it is light and fun. Even so, it is a good read.