Murder on the Astor Place by Victoria Thompson

The first novel in the national bestselling Gaslight Mystery series introduces Sarah Brandt, a midwife in the turn-of-the-century tenements of Manhattan who refuses to turn a blind eye to the injustices of the crime-ridden city…
After a routine delivery, Sarah visits her patient in a rooming house—and discovers that another boarder, a young girl, has been killed. At the request of Sergeant Frank Malloy, she searches the girl’s room. She discovers that the victim is from one of the most prominent families in New York—and the sister of an old friend. The powerful family, fearful of scandal, refuses to permit an investigation. But with Malloy’s help, Sarah begins a dangerous quest to bring the killer to justice—before death claims another victim...