A Certain Age: HarperCollins

As the hedonism of the Jazz Age transforms New York City, the iridescent Mrs. Theresa Marshall of Fifth Avenue and Southampton, Long Island, has done the unthinkable: she’s fallen in love with her young paramour, Captain Octavian Rofrano, an aviator and a hero of the Great War.
Though the battle-scarred Octavian is devoted to his dazzling socialite of a certain age and wants to marry her, Theresa resists. The old world is crumbling, but divorce for a woman of Theresa’s wealth and social standing remains a high-stakes affair. And there is no need: she shares a gentle understanding with Sylvo, the well-bred philanderer to whom she’s already married.
That is, until Theresa’s impecunious bachelor brother, Ox, decides to tie the knot with Miss Sophie Fortescue, the naïve young daughter of a wealthy inventor. Theresa enlists Octavian to check into the background of the reclusive Fortescue family. When Octavian meets Sophie, he falls under the spell of the charming ingénue, even as he uncovers a devastating family secret.
As a fateful triangle forms, loyalties divide and old crimes are dragged into daylight, drawing Octavian into transgression . . . and Theresa into the jaws of a bittersweet choice.