The Traitor in the Tunnel by Y.S. Lee
The third mystery in The Agency series finds Mary Quinn working undercover as a maid at Buckingham Palace. Someone has been stealing from Queen Victoria. While investigating, Mary must fend off the Prince of Wales. She also once again works with James Easton who reacted poorly when he heard about her criminal past in book two.
This series combines historical details with intriguing mystery. In this book, Mary's character grows as she deals with secrets from her past and her evolving feelings for James.