An Extraordinary Union by Alyssa Cole

An engrossing historical romance featuring spies during the American Civil War. Born into slavery, Elle Burns is now a free woman. In 1862, she goes undercover, working as a slave in the household of a Confederate senator in Richmond, Virginia. She meets Pinkerton Secret Service Agent Malcolm McCall, a Scottish immigrant, and the pair form an uneasy alliance. As their relationship (both professional and personal) grows, Elle and Malcolm must navigate the uneasy world of race, politics, and war. 
Although darker in tone, I'd recommend An Extraordinary Union (2017) by Alyssa Cole to fans of Lauren Willig's The Secret History of the Pink Carnation--both feature an irresistible combination of intrigue, history, adventure, and romance. It’s also great for fans of Beverly Jenkins’ richly detailed historical romances. And if you enjoy the novel, it’s the first in the Loyal League series: the next two entries are A Hope Divided and An Unconditional Freedom.