She Came to Stay, by Simone de Beauvoir

Set in Paris on the eve of World War II and sizzling with love, anger, and revenge, She Came to Stay explores the changes wrought in the soul of a woman and a city soon to fall. Although Françoise considers her relationship with Pierre an open one, she falls prey to jealousy when the gamine Xavière catches his attention. The moody young woman from the countryside pries her way between Françoise and Pierre, playing up to each one and deviously pulling them apart, until the only way out of the triangle is destruction.