Vivian Maier is the Emily Dickinson of street photography. Maier worked as a nanny in New York and Chicago for more than a half-century, and took thousands of brilliant photographs, never sharing them with anyone except, occasionally, for the children in her charge. Some of her negatives were found by chance, when a local real estate agent purchased them at an auction house; now an exhibition of her photography is traveling the world (this summer it's in the U.K.)....