The timing, composition and realisation grab you. But it's the anonymous drama of the subjects that keeps you looking: who are the two men talking in earnest on the pier? What is running through the mind of the young black man caught in the mirror on a street corner? Yet in the case of the photos presented here, that mystery extends to the photographer herself. We know she was called Vivian Maier, that she took at least 30,000 shots in Chicago and New York in the 1950s and 1960s. For these few facts, we can thank a young Chicago estate agent called John Maloof, whose chance discovery of the Maier archive has brought publishing offers, invitations to exhibit Maier's work all over the world, and the hope, from some, that a new star of mid-century American photography may have been unearthed.

A couple of years ago, Maloof wasn't even that interested in photography. With real-estate business slow due to the credit crunch, he began to investigate a box of negatives he'd bought for a few hundred dollars at an auction as a possible source of archive photography for a neighbourhood history book. Maloof scanned a few of the negatives, liked what he saw, and bought himself a beginner's SLR camera. "Throughout that time, I'd compare my work to Vivian's and think, 'Wow, this isn't good for me.' She was teaching me photography. I bought some books by street photographers such as Harry Callahan, Henri Cartier-Bresson, Lee Friedlander. I thought, she's special – doesn't anybody know?"

Nobody did, it seemed. Maloof had gleaned Maier's name but little else from the intact but uncatalogued archive. He Googled her name without luck, and went so far as to track down another box of her work that a "found photography" enthusiast had bought at the auction. By which point he had 20,000 negatives and about a thousand rolls of film with 12-14 images on each – but no more clues as to the young, reserved-looking woman occasionally seen peering into shop windows or mirrors for a self-portrait, Rolleiflex camera around her neck.

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A call to the auction house gave Maloof hope: Maier was alive, he was told, but believed to be too ill to receive visitors. Come May this year, Maloof had set up a blog displaying what he thought to be the best of the images so far (he still updates www.vivianmaier.com most days). He Googled again, only to discover a Chicago Tribune obituary dated a few days earlier: "Vivian Maier, proud native of France and Chicago resident for the last 50 years died peacefully on Monday. Second mother to John, Lane and Matthew. A free and kindred spirit who magically touched the lives of all who knew her... Movie critic and photographer extraordinaire." She had died at the age of 83.

The newspaper directed Maloof to an address in a Chicago suburb, but he was able to turn up nothing. Meanwhile, Maloof joined Hardcore Street Photography, a group on Flickr, the photo-sharing website, wondering if his enthusiasm for Maier's work would be shared by others. "That week it exploded, and my inbox really filled up," says Maloof. "I've been asked to display these photos in Australia, Canada, UK, France and Mexico City."

So far, Maloof has put up a few hundred images on his blog, and he reckons, by his inexpert eye, that about one in 10 of Maier's images is worth posting –there are at least 20,000 images that he hasn't even looked at yet. No wonder he admits to feeling overwhelmed by the project of exploring Maier's huge archive. '

From what has emerged so far, it's easy to see what has excited Maloof and others worldwide. The well-to-do shoppers of Chicago stroll and gossip in all their department- store finery before Maier, but the most arresting subjects are those people on the margins of successful, rich America in the 1950s and 1960s: the kids, the black maids, the bums flaked out on shop stoops. There are more formal, almost abstract experiments with light and line, too. "She has an amazing appreciation of light. You can see it developing through her work," says Maloof.

Then, a couple of weeks ago, Maloof got lucky. He came across an address on a label in the archive. He discovered that two people with the Christian names Matthew and Lane were registered there. One phone call later, and he learnt that Vivian Maier had been a nanny to John, Matthew and Lane for a decade from the early 1950s in a wealthy suburb of Chicago. And at a lunch with two of her charges, now in their fifties, he learnt much more.

A French Catholic, Maier had apparently arrived in New York as a young girl in the 1930s, where she had earned her keep in sweatshops and learnt English at the theatre. Eventually, she found herself in Chicago nannying three boys. "She had a peculiar personality," Maloof was told. "They told ' me she would bring home a dead snake to show them, or convince the millkman to drive them all to school in his delivery truck. They loved her." She had no family that anyone knew of, never, so it's said, taking a single personal call at the house she worked in for a decade. "She wore big hats and coats, and men's shoes, and thought of herself as a film critic." Her camera was around her neck constantly. As the children grew up, Maier moved on to nanny other families. But by the 1990s, she was homeless, and fortunate that the three boys she had originally looked after were able to return the favour, buying her an apartment and paying her bills until she died.

Why did this intensely private person pursue her rich, revealing photography? Certainly not for public show believes Maloof, who says there only a few, small prints in the archive. Who was Vivian Maier? Perhaps more biographical tidbits will emerge over the next few years – but there is surely only one place where we might find the answer: in the dark room.