In this exquisitely crafted psychological thriller from Edgar finalist Unger (The Stranger Inside), Selena Murphy trades confessions with a stranger named Martha while the two are trapped in a stalled New York City commuter train. Martha admits that she's having an affair with her boss, and Selena tells Martha that she suspects her husband, Graham, is sleeping with their nanny, Geneva. In truth, Selena knows Graham and Geneva are having sex thanks to a surveillance camera she placed in the playroom, and sends Graham packing when she arrives home. Selena resolves to fire Geneva, but then Geneva's sister reports her missing, forcing Selena to question just what her husband is capable of. Compounding her anxiety are texts from Martha, to whom Selena never divulged a last name or phone number. Unger explores the complexities of marriage and the devastation wrought by infidelity using keenly rendered characters and a prismatic narrative. Though Geneva's disappearance drives the plot, smaller puzzles abound, heightening suspense and connecting seemingly disparate story lines in diabolically clever ways. Unger just keeps getting better. Agent: Amy Berkower, Writers House. (Oct.)