The influential feminist author known for her book "The Second Sex" is Simone de Beauvoir. Published in 1949, this seminal work critiques the historical and social constructs surrounding gender, establishing de Beauvoir as a leading figure in feminist thought. In "The Second Sex," she famously asserts, "One is not born, but rather becomes, a woman," challenging traditional views of femininity. De Beauvoir's ideas have significantly shaped feminist theory and continue to resonate in discussions about gender identity and equality.