Rules of Civility by Amor Towles

On the last night of 1937, twenty-five-year-old Katey Kontent is in a second-rate New York City jazz bar trying to stretch three dollars as far as it will go. But a chance encounter with the handsome banker at the next table changes everything, opening the door to the upper echelons of New York society and a glittering new social circle. Plunged into a dizzy world of cocktail parties, sprawling mansions and glamorous magazine offices, Katey soon learns that there are rules to play by and riches can turn to rags in the trip of a heartbeat.