An unexpected treat! There are many kinds of poisons, but back in the early 20th century, there wasn't much knowledge about them. Often it was the case that a substance wasn't even known to be poisonous. Other poisons were known about but it wasn't known how to measure them or assess their action.
It is written with a deceptively breezy style: there's a fair amount of science hiding in there but you barely notice it because the book is heavily laced with tales of nefarious doings and dastardly crimes, as well as tragic stories of ignorance leading to unexpected deaths. Mercury-laden tonics, ubiquitous arsenic distribution, beauty creams fortified with radium or thallium. Cyanide, mercury, carbon monoxide poisoning -- they all were deadly compounds attached to a roster of fascinating stories. But the dogged work of two uncaped crusaders of New York City -- Norris and Gettler, the first medical examiners and toxicologists -- helped create and define the field of forensic medicine. They brought science in to the light to show it could be used to solve crimes. They also tirelessly worked to demonstrate that Prohibition was deadly. It resulted in replacing legal alcohol with toxic alcohol alternatives, and blindness, paralysis and deaths skyrocketed.
They were public service heroes, battling corrupt politicians, lazy bureaucrats, public apathy, and venal greed.
Some battles are never won. Some things never change.