Through a Glass Darkly — Donna Leon

On a luminous spring day in Venice, Commissario Brunetti and his assistant, Vianello, play hooky in order to help Vianello's friend Marco Ribetti, an environmental activist who has been arrested during a protest. But on the steps of the Questura, they come face to face with Ribetti's cantankerous father-in-law, Giovanni De Cal, who has been overheard in the bars of Murano making threats against Ribetti. When a body is found in front of a blazing furnace at De Cal's glass factory along with an annotated copy of Dante's Inferno, Brunetti must investigate. Does the book contain the clues he needs to solve the murder and uncover who is ruining the waters of the lagoon?