Emily St. John Mandel’s new novel opens in the first spring after the collapse of the United States – and her protagonist, Ari, is on her way to a party. Exit Party is set in a new world emerging out of the chaos of a second American Civil War, one shaped by climate catastrophe and populated by doppelgangers. In today's episode, Mandel chats with Weekend Edition’s Scott Simon about counterlives, recognizing “good moments” amid disaster and how t…