Princess Elizabeth's Spy by Susan Elia MacNeal

In this second book of the Maggie Hope series, Maggie is sent to Windsor Castle by MI-5. Her mission is to help protect Princess Elizabeth, but she will also be working as Elizabeth's math tutor. The British authorities feel the princess might be in danger, since she is next in line to the throne. Soon after Maggie arrives, one of the Ladies-in-Waiting is murdered and she wonders who among the castle employees is loyal to the Nazis. Princess Elizabeth's Spy is another enjoyable entry in this mystery series, with lots of details about life at Windsor Castle during World War II.