Girl in Disguise by Greer Macallister (2017) Sources Landmark

A history of the first woman to be a Pinkerton detective.  As a widow, Kate Warne is out of options and refuses to be pigeon-holed into what society expects of her.  Instead, she solves a crime and uses that success to get Pinkerton to hire her.  She fluidly goes through disguises to work whatever case she is assigned.  But she uses her mind too, and to great effect and the consternation of those she has to catch.