The book starts out as a "street urchins in Victorian London" story. Is it going to be another Dickens book? Luckily, it is not. The boy associates with a doctor who is trying to find the cause of a cholera epidemic. The prevailing theory is the "bad air" miasma cause of disease. However, the doctor thinks it may be due to water. The boy helps identify those that have died and record the details. He even locates somebody that lives far from the pump that died - after they had water from the pump. The solution seems to be to take the handle off the pump to prevent people from using it. However, it is a popular source for clean water, so they need solid proof. Unfortunately, the boy ran into his step father, who tied him up. Luckily, he escapes and all goes well. They later identify a baby that had been one of the early cases. Their waste had been dumped in a cess pool that had leached into the nearby well.
The book has a great mix of the tale of scientific discovery combined with the life of an everyday person that lived it. There is even a "fish out of water" ending, where the boys later father had worked recoding data for the government - just like the boy likes to do.
Back in the days, people could carry out long term letter-writing relationships. The slow pace of the communication allows for a more deep level of contact than is present in the more instantaneous forms of communication.