Seamless ... Weakness for catchphrases is my least favorite thing about Exit Party and the Mandelverse in general. It speaks to an impulse to reduce the novels’ mysteries to accessible bromides. Still, the habit is in keeping with the book’s paradoxically optimistic tenor. Ari and her counterpart are smart, resilient, personable women who, for all their wrongdoing, have a gift for finding happiness in the midst of breakdown. The hope they represent is the stabilizing force that bridges Ms. Mandel’s enterprising fiction.