Bestseller McFadden's jaw-dropping latest (after The Housemaid Is Watching) keeps the author's trademark twists coming fast and furious. Thirty-four and unhappily single, Sydney Shaw haunts the popular dating app Cynch, where she matches with a string of losers. One night, while Sydney fights off the unwanted advances of her latest date, a handsome stranger comes to her rescue. He turns out to be a doctor named Tom Brewer, and soon he and Sydney strike up a relationship. All goes well until Sydney's neighbor turns up dead, and her ex-boyfriend, homicide detective Jake Sousa, is assigned to the case. Jake links the murder to a string of killings across New York City--many facilitated by the use of Cynch--and Sydney starts to worry that her new boyfriend is hiding something deadly. McFadden toggles between Sydney's perspective in the present and Tom's in the past, while he carries on a relationship with a woman named Daisy. It all comes together with a series of hard-to-predict but fair-play surprises that reward rereading. McFadden extends her hot streak with this canny exploration of love, death, and dating. Agent: Christine Hogrebe, Jane Rotrosen Agency. (Oct.)