The Second Sex — Simone de Beauvoir (1949)

Simone de Beauvoir's The Second Sex is the foundational text of second-wave feminism, providing the philosophical framework for understanding gender as a social construction ("One is not born, but rather becomes, a woman") rather than a biological destiny. Its exhaustive analysis of how patriarchal society constructs femininity through mythology, biology, history, and literature remains one of the most ambitious works of feminist philosophy ever written.