Those who like to pillory the Victorians do so for their application of a double standard, licentiousness in private while appearing spotlessly upright in public. Callander Square is a powerful commentary on that double standard, as the story strips away the cloaks of respectability among neighbors in an upper class neighborhood.
Upper class lives were then seldom examined . . . except by ladies who were gossiping. When two dead babies are found by accident buried in Callander Square, it becomes Inspector Thomas Pitts' duty to examine all of those lives . . . looking for who the mother was. Pitts' theory is that if you can find the mother, you can find the murderer . . . or the circumstances of death if it wasn't murder.
The wealthy men and women in the square do their best to fend off Pitt by focusing him on their servants. Unsuspected by them, Pitts' wife, Charlotte, decides that she wants to find the mother too . . . but to succor rather than to accuse her. Charlotte and her sister Emily play an undercover role in which Emily is the Upstairs mole and Charlotte is the Downstairs mole. Soon, the skeletons are rattling in all the relevant closets. And crimes multiply!
This mystery presents an interesting problem. How do you investigate when all the "good" people either won't talk to you . . . or lie when they do? These people are so delicate that they won't even come out and discuss their concerns. One has to hint around . . . and hope that the message is received and understood. So there's a dance of manners involved here inside of a mystery which is inside of a dysfunctional society. For those who like novels of manners, there is much to enjoy here in addition to the mystery.
I give Ms. Perry great credit for hiding the villains until late in the book. You will know in the last 80 pages or so who did what, but it's a totally incomprehensible mystery before then. If she had shortened up the end a bit, I would have graded the book higher. But the climax is more like a tea party that's gone on too long than a climax until the last few pages.
The writing is superb. A large number of characters are fully developed, and the development is used well to advance the plot.