Ultimately, the issue with The Suspicions of Mr. Whicher is that Summerscale juggles enough material for three different books without proper integration. There is the biographical portion on the great Victorian detective, Jack Whicher, an account of the Road Hill Murder, and an investigation of how both Whicher and the Road Hill case directly affected what was to become known as "detective fiction". All three aspects are interrelated and highly complimentary. They even play together nicely in the blurb - hence my excitement, but they compete with each other in the book, where Summerscale jaggedly juxtaposes them along with other semi-related facts (Protestant distrust of the Roman Catholic Church and watercolors of the Great Barrier Reef being some of the larger tangents).
The Whicher biography was a compelling portrait of a sharp-minded man ahead of his time, ruined by the largest case of his life. Summerscale handles the Road Hill portions with equal interest, painting the scene with forensic detail and compelling emotion. She also makes some genuinely interesting connections between the Whicher/Road Hill historical material and the literary craze that it spawned. One can definitely see Whicher as the prototype for Inspector Cuff in The Moonstone by Wilkie Collins. There are even shades of Sherlock Holmes in his single-minded pursuit of forensic proof. Likewise, one can easily see how the Road Hill case spawned hundreds of literary imitations in the form of the "estate mystery," where a murder or crime takes place at a large country manor, with the guilty party being either family or visiting friend. Every single aspect of the book is interesting, but therein lies the problem. They each remain single aspects instead of well-integrated parts of a whole.
Given all of this, I recommend the book but I do so with a warning: The narrative and historical fascination Summerscale delivers are tempered by structural flaws and tangential wandering. I recommend reading the parts that interest and skimming over the rest to avoid what I found to be irritating side-tracks.