The Detective's Assistant

From author Kate Hannigan's website:

Based on the extraordinary true story of America's first-ever female detective, this fast-paced adventure recounts feats of daring and danger...including saving the life of Abraham Lincoln!

Eleven-year-old Nell Warne arrives on her aunt's doorstep lugging a heavy sack of sorrows. If her Aunt Kate rejects her, it's the miserable Home for the Friendless.

Luckily, canny Nell makes herself indispensable to Aunt Kate...and not just by helping out with household chores. For Kate Warne is the first-ever female detective employed by the legendary Pinkerton Detective Agency. And Nell has a knack for the kind of close listening and bold action that made Pinkerton detectives famous in Civil War-era America. With huge, nation-changing events simmering in the background, Nell uses skills new and old to uncover truths about her past and solve mysteries in the present.