Series numeration from goodreads.com

Detective John Hooper, William Monk's right-hand man at the Thames River Police, is blissfully happy in his new marriage to Celia, the cousin of a victim in one of the river police's recent murder cases. Celia wants the same happiness for her good friend Clementine, who's just announced her engagement to Seth Marlowe, a fellow member of her church. Christmas is nearing, and this should be extra cause for celebration, but when Marlowe begins receiving threatening letters about his first wife's death, it becomes clear that he is far from the devout man Clementine thought he was. In his rage, Marlowe accuses Celia of sending the letters, claiming she wants to ruin his engagement to Clementine. At a loss as to how to defend herself, Celia enlists her husband to investigate the letters' claims, and what he finds makes her desperate to show Clementine the truth about her intended. But Celia herself has not always been truthful, especially not during the murder trial following her cousin's death. How can she be believed now, when she lied on the stand? Especially when Marlowe knows that she did, and could use it against her. This Yuletide season finds love and faith put to the test - and Celia's and Clementine's lives on the line.