Throughout the 10 stories in her first collection of short fiction, Curtis Sittenfeld upends assumptions about class, relationships and gender roles in a nation that feels both adrift and viscerally divided. In “The World Has Many Butterflies,” married acquaintances play a strangely intimate game with devastating consequences. In “A Regular Couple,” a high-powered lawyer honeymooning with her husband is caught off guard by the appearance of the girl who tormented her in high school. And in “The Prairie Wife,” a suburban mother of two fantasizes about the downfall of an old friend whose wholesome lifestyle empire may or may not be built on a lie.