As the Detroit Tigers and Chicago Cubs faced off in the World Series, photographer Maurice Terrell arrived at an almost deserted minor-league park in San Diego, California, to carry out a top-secret assignment: to surreptitiously photograph three black baseball players.

Terrell shot hundreds of motion-picture frames of Jackie Robinson and the two other players. A few photos appeared in print but the existence of the additional images remained unknown for four decades. In April 1987, as Major League Baseball prepared a lavish commemoration of the fortieth anniversary of Robinson’s debut, I unearthed a body of contact sheets and unprocessed film from a previously unopened carton donated in 1954 by Look magazine to the Baseball Hall of Fame in Cooperstown, New York....