An Extraordinary Union by Alyssa Cole

Set during the American Civil War, the story follows Elle Burns, a former slave with an eidetic memory, who works as a spy for the Union. Disguised as a mute slave in the Confederate South, she gathers crucial intelligence while navigating the dangers of her mission. Her path crosses with Malcolm McCall, a Scottish Pinkerton detective also undercover, and despite the risks, they develop a deep, romantic bond. Together, they face the challenges of espionage, loyalty, and love in a time of intense conflict and societal upheaval. The 7501st greatest book of all time