Meticulous books on the city (I recommend "City: Rediscoving the Center (1988)"). With the help of graduate students he did many studies on exactly how people used open spaces, plazas street corners. Some of his findings: in a stream of pedestrians friends talking tend to stand in the most congested spot; most street parking is cars that have been there for 6 hours or more; corporations that move to the suburbs are about half as profitable as those that stay intown.