“Although initially terrified of the piers,” the Bronx-born photographer Alvin Baltrop wrote in a book he never finished, “I began to take these photos as a voyeur [and] soon grew determined to preserve the frightening, mad, unbelievable, violent, and beautiful things that were going on at that time.” It was the 1970s and 80s—post Stonewall and pre AIDS—and the Hudson River piers on Manhattan’s lower west side were a scuzzy Shangri-la for nude sunbathing and cruising. Baltrop died of cancer in 2004, at age 55. The international fame these photographs brought him came posthumously. —L.J.