Who Cries for the Lost by C.S. Harris is a gripping addition to the historical mystery genre, featuring the enigmatic and complex protagonist Sebastian St. Cyr, Viscount Devlin. Set against the tumultuous backdrop of Regency England, this latest installment dives deeper into the labyrinth of London’s aristocracy, blending deftly woven narratives of intrigue, betrayal, and justice. Harris's narrative is both lush and evocative, transporting readers to a time of both exquisite beauty and brutal disparity.

The novel opens with a scene that sets the stage for suspense and drama. A young boy, witnessed by Sebastian, is shoved into the path of an oncoming carriage and killed. This incident is not merely a hit-and-run but is shadowed with darker undertones of aristocratic conspiracy and secrets that perhaps demand too high a price for their keeping. The deceased boy is soon revealed to have ties to Sebastian’s own convoluted past, a past that consistently haunts and challenges him in new and unexpected ways.

Sebastian St. Cyr, as crafted by Harris, is a protagonist of remarkable depth and compelling moral compass. His aristocratic background affords him certain privileges, yet it is his disregard for high-society’s dispensations in the face of justice and truth that renders him a hero of a very distinct badge. His investigations take him into the dark underbelly of a society rigidly divided by wealth and birthright, where he encounters characters drawn in shades of moral ambiguity and complexity. It is these characters that enrich the narrative, adding layers and voices that speak of the era's harsh realities, complicated further by personal vendettas and political maneuverings.

Harris does an admirable job of exploring Regency England with an eye that is both critical and sympathetic. Through descriptive passages rich with historical detail, she captures the stark contrasts between the opulence of the aristocracy and the grim despair of the less fortunate. Her depiction of London is particularly noteworthy; it serves not only as a setting but almost as a character itself—a city pulsing with life, danger, and possibilities. The thorough research Harris has committed to her setting shines through in her meticulous attention to historical accuracy, from the dialogue to the customs and social contexts of the time.

The mystery itself is tightly plotted, with twists that are unpredictable yet plausible. Harris weaves multiple threads and characters into her story, managing them with skilled narrative control so that the resolution is both satisfying and logically consistent with the story’s development. The pacing is adeptly managed, with tension building steadily through revelations and reversals that keep the reader deeply engaged and guessing at the true breadth of the conspiracy. The integration of real historical figures and events lends an additional layer of authenticity and interest to the story.

However, it is not just a tale of murder and intrigue. At its heart, Who Cries for the Lost also deals with themes of family, loyalty, and the conflicts between personal desires and moral duties. Sebastian’s personal growth and his relationships, particularly with his wife, Hero, add an emotional depth to the novel that extends beyond the boundaries of the mystery genre. Harris handles the growth of her characters with finesse, allowing them to develop in ways that are both true to their personas and indicative of the novel's larger thematic concerns.

One of the notable aspects of Harris's writing is her ability to convey emotional subtlety and psychological complexity. The dialogue is often sharp and revealing, offering windows into the characters' inner lives without detracting from the narrative movement. Moreover, the moral conundrums presented are neither superficial nor easily resolved, challenging both the characters and the reader to reconsider notions of justice and retribution.

In conclusion, Who Cries for the Lost by C.S. Harris is a masterful blend of historical accuracy, complex characterization, and riveting mystery. It stands not only as a thrilling narrative but as a deep dive into the conflicts and machinations of Regency England. For fans of historical mysteries, this novel promises and delivers on many fronts—be it the thrill of the chase, the depth of historical immersion, or the pleasure of navigating the murky waters of human motives and misdemeanors. This book is highly recommended for its clever plotting, its atmospheric setting, and its unwaveringly thoughtful prose.