The Game She Plays by Siena Sterling

From the author of  Tell Us No Secrets  comes a twisty new novel. A young American woman is thrust into the closed social circle of English aristats, but beneath their charm and glamor lay secrets and dark betrayals that follow them—and her—around every corner. Perfect for fans of Lucy Foley, Sherry Lapena, Ruth Ware, and Gilly MacMillan. 
To everyone else it was "just" a weekend house party—complete with a pheasant shoot, inside jokes, and after-dinner games. But for American Nicola Harris, swept off her feet by aristatic Englishman James Shuttleworth, the weekend is her first chance to penetrate his close-knit circle of posh friends—people with names like Badger and Trez, people who will soon have the titles of Earl and Countess. Nicola is eager to fit in, but she can't seem to navigate this insular world, where everyone speaks a private language and secrets are hinted at behind the jolly jokes and witty barbs.
Just as Nicola begins to feel welcome, Juliet arrives. Mysterious, stunning, charismatic Juliet is also James' first love—the one who left him heartbroken ten years before. Juliet holds the men in thrall and the women under her spell. And although it seems she can do no wrong, outsider Nicola senses something poisonous in her presence, something that sparks insecurity, jealousy, betrayal, and violence. Because Juliet is a champion game-player, able to lure you into her circle by finding your most vulnerable spot....and when the game she plays turns deadly, everyone is a suspect.