The Widow Spy by Megan Campisi

It's 1861 and Kate Warne and several of her Pinkerton colleagues are trying to break Confederate spy Rose Greenhow. They are searching Greenhow's home for her cipher key, because finding it would help the Union's cause immensely. Greenhow, however, is clever and threatens to derail the group's plans. In The Widow Spy, Campisi details the life of Kate, who was a real person. An interesting read for those who enjoy historical fiction featuring female characters.