Lady Emily Hargraves grew up with Emma Callum. Their mothers were friends, and Emma made Emily's life miserable. Emma eventually scandalized society by eloping with a Venetian Comte, and as the story opens, she's showing Emily and her husband Colin the jeweled and blood-encrusted dagger found in her father-in-law's chest. Emma can't (or won't) trust the local authorities, so calling Emily is her only option. Emily agrees, and soon meets scholar and bookseller's daughter Donata Caravello. Donata serves as Emily's guide through Venice, teaching her the social rules and multi-generational feuds of Venice. Interspersed with Emily's 19th Century investigations are chapters of Besina Barozzi's and Nicolo Vendelino's 16th Century romance. Though passionately in love (with each other and with Dante), they belong to feuding families so their love is clearly doomed.
Alexander sprinkles enough clues in both the main story and the historical romance for the reader to solve the mystery shortly before the killer is revealed, so the book works as a mystery. Death in the Floating City shines, however, as a novel. The author's descriptions of Venice made me not only want to visit but to time travel so I could see what Emily saw, and there's some interesting commentary in the four romances (two "society" and two "doomed") portrayed in the book. She also provides a bittersweet ending where the killer faces justice and a centuries long feud ends, but Emily once again faces a loss.