Thompson's fourth Gaslight Mystery presents convincing characters of varied classes of a century ago, when crime flourished throughout New York. Detective sergeant Frank Malloy, a widower and Irish, finds himself forced to let midwife Sarah Brandt, a widow and Knickerbocker, assist in his investigations. A young woman who was blackmailing vulnerable men is stabbed and dies near the square's hanging tree. The owner and residents of her boarding house soon come under suspicion, and someone tries to kill an inquiring news reporter. Frank and Sarah and some nosy neighbors follow the many twists of the plot with dogged and sometimes close to foolhardy determination. There are several clever but fleeting scenes of the policeman and the midwife realizing their relationship could be more, and this reviewer suspects the author will continue this state of suspended animation between two such appealing opposites into the next book in the series. Older YAs and adults will be challenged to solve the mystery and find, as the author declares in her concluding note, that putting the victim on trial is a tactic that continues today. (A Gaslight Mystery).