The elements of murder
I was surprised at how long it took to catch some of the murderers and I'm glad that modern procedures are much better at identifying these poisons.
I found that this book takes a bit of getting into - its a long book, and murders which would merit a few pages elsewhere get a whole chapter here. However, once you accept this then you should find it an enjoyable read.
There is also speculation on how some well know people may have been affected by these poisons. Newton probably suffered from mercury poisoning due his alchemical studies. The madness of King George could have been caused by lead in his wine, and the antimony in Mozart's medicines probably killed him.