The Mitford Vanishing by Julian Fellowes

Set in the interwar English countryside, this atmospheric mystery follows a wealthy aristocratic family when a beloved young member disappears from their grand estate during a weekend gathering. As an outsider pulled into the household peels back polite surfaces, long-buried rivalries, wartime trauma and illicit romances come to light, revealing that the family's genteel façade conceals resentments, secrets and dangerous loyalties. A tense unraveling of motive and memory forces reckonings about class, privilege and the shifting social order of the 1920s.