A few years ago, John Maloof was looking for old photos for books he was writing on Chicago's neighborhoods. On a hunch, he bought an auction lot of 30,000 negatives. They were the unknown photos of a woman who'd worked as a nanny for 40 years on the city's North Shore and had spent all that time making photographs. Vivian Maier's images are now being called some of the most important street photography of the 20th century. We hear more from Maloof, who now owns more than 100,000 of Maier's negatives, and from Lanny Silverman, chief curator at the Chicago Cultural Center, where a major show of Maier's work opens next month.