His most poignant series of photographs? Those of his wife and her three siblings. Begun in 1975, the series of group portraits titled "The Brown Sisters" features the same four women arranged almost identically in 33 frames taken, one each year, across 33 years. In an exhibition of large-scaled, beautifully-printed, black-and-white photographs, the portraits have traveled the country, with stops at the Museum of Modern Art, National Gallery of Art, the Cincinnati Art Museum, and other venues. Now, they've also been collected in book form by the Museum of Modern Art (Nicholas Nixon: The Brown Sisters. Thirty Three Years; $34.95; www.momastore.org).