There are artists whose day jobs had nothing to do with what made them famous in their lifetimes (the poet Wallace Stevens was an insurance executive, and Anthony Trollope worked in a post office), but rarer still is the one who toils in anonymity and whose talent is discovered and widely celebrated only after her death. Such is the story of Vivian Maier, a nanny for decades who in her spare time took tens of thousands of photographs, mostly of ordinary people on the streets of New York and Chicago.

The tale of how a cache of her negatives, purchased in a warehouse foreclosure sale, initiated a search for the woman whose name was on the boxes is well known. But what Ann Marks does so brilliantly in her forthcoming book, Vivian Maier Developed, is to discover the singularly private woman behind the camera and the tragic events that shaped Maier and led her to become one of the 20th century’s premier street photographers. What makes the book such a revelation is how Marks matches biography with photographs, some 400 images, many never published before, that reflect the mind of a troubled genius.

Vivian Maier Developed: The Untold Story of the Photographer Nanny, by Ann Marks, will be published on December 7 by Atria