This is a wonderfully done true crime story of a murder in England in 1860. If that were all, we'd have an eminently enjoyable book. But this is also a social commentary and a history of the early detective story: you'll learn how and when the words "clueless" and "sleuth" entered the language, for example. You have a horrible murder of a 3-year-old boy in a manor house in the country. The outside doors, windows, and gates are all locked--and also, unusual for us nowadays, many of the interior doors were locked as well--preventing access to the larder, cellar, drawing-room, etc. So suspicion perforce falls upon the family and servants. This is before the days of forensic science--so it isn't even clear whether the child was killed by stabbing, throat-cutting, suffocation, or drowning. The local constabulary in this west England area are inadequate to the task in what very quickly becomes a sensationalist case, and so a detective from London is called in to investigate.
Detectives are new, only a couple of decades old, as are detective stories. Detective-Inspector Jonathan Whicher is Scotland Yard's best investigator (at the time, there weren't all that many). The child's family is not very well liked in the area, and the family itself has many unsavory secrets--including insanity. Summerscale relates Whicher's detective work and his growing fixation upon a 16-year-old sister. But what makes all of this particularly enjoyable is how Summerscale relates the sensationalism in the press, the plethora of theories as to the murder, the coming-forth of outsiders to confess, the initial belief in Whicher's abilities (followed by growing disbelief). There are wonderful descriptions of the detective novels of the time--including ones with female detectives--the public appetite for these stories, and the additions to the language (you'll see where clue/clew comes from). The child's nanny slept in the room with the child, who was taken during the night. Charles Dickens was one of the numerous people who put forth the theory that the child had discovered his father in bed with the nanny and had been killed to prevent him telling Mama. Actual solutions, however, were not readily forthcoming.
Whicher fell out of favor in the public eye--but he did pop up again in the other sensational case of the era--the Tichborne Claimant. (Hopefully, Summerscale will turn her prodigious talents to that case next). So what you get here is a fascinating view of the early days of detectivedom (if that's a word), the detective in fact and fiction, and the public's taste in literature. The book reads like a good detective novel, with well-portrayed characters: there are arrests, trials, maps, drawings, and photographs. A great book indeed!