Ugochukwu-Smooth C. Nzewi, the Steven and Lisa Tananbaum Curator of Painting and Sculpture, looks at Sue Williamson’s “For Thirty Years Next to His Heart,” in which the 30-year life of one man’s official government passbook “captures the experience of Black South Africans under apartheid”—and serves as a reminder that we must not forget the most difficult chapters of our history.