The Case of the Missing Maid by Rob Osler

In 1898 Chicago, Harriet Morrow, a bicycle-riding, trouser-wearing lesbian, becomes the first woman field operative at the Prescott Detective Agency. Her first assignment is to find a maid who has vanished from a Prairie Avenue mansion, a search that takes her into the city's Polish community, brings her into a friendship with the maid's older sister, Barbara, and uncovers dangers she did not expect.