The Alienist by Caleb Carr
Summary from Goodreads : The year is 1896, the place, New York City. On a cold March night New York Times reporter John Schuyler Moore is summoned to the East River by his friend and former Harvard classmate Dr. Laszlo Kreizler, a psychologist, or "alienist." On the unfinished Williamsburg Bridge, they view the horribly mutilated body of an adolescent boy, a prostitute from one of Manhattan's infamous brothels. The newly appointed police commissioner, Theodore Roosevelt, in a highly unorthodox move, enlists the two men in the murder investigation, counting on the reserved Kreizler's intellect and Moore's knowledge of New York's vast criminal underworld. They are joined by Sara Howard, a brave and determined woman who works as a secretary in the police department. Laboring in secret (for alienists, and the emerging discipline of psychology, are viewed by the public with skepticism at best), the unlikely team embarks on what is a revolutionary effort in cr