THE PHOTOGRAPHS are clear – immediate and vital, intimate or journalistic, composed with great sensitivity whether on manicured lawns or down garbage-strewn alleyways – yet the life is a mystery. Vivian Maier (1926-2009) worked as a nanny in the Chicago area who always carried a camera, usually a Rolleiflex, slung around her neck. Through decades of service, she captured more than 100,000 images on the street or at her employers’ homes, shot thousands of rolls of film and recorded her thoughts on audio cassette. She kept all these, along with many other artefacts, newspapers and more, strictly to herself, insisting that her room be fastened with a stout lock. John Maloof is a young historian and ­photographer who seven years ago, while compiling a book about Chicago,