In his short but illuminating introduction to this epic three-volume retrospective of his work, Bruce Davidson recalls the pivotal moments in a career that stretches over 50 years. In 1943, aged 10, in Oak Park, Illinois, he visited a darkroom in his friend's house and was lost "in boyhood wonderment" at the mysterious process he witnessed. He went home and turned a cupboard into a makeshift darkroom.

Soon afterwards, his father bought him a $4 Falcon 127, which he used until, at 15, he was given an "expensive Kodak Medalist camera" by his stepfather, and won the Kodak High School Snapshot contest for his portrait of "a baby owl in a trail-side museum in the woods near our house".

At college in Rochester, Illinois, Davidson met a beautiful girl called Joan, who owned a copy of The Decisive Moment by Henri Cartier-Bresson. "It was then I fell in love for the first time with the girl and with Cartier-Bresson."

Having relocated to New York in the early 1950s, Davidson worked for Eastman Kodak, photographing "banal objects", and applied to Yale. The artist Josef Albers reviewed his portfolio of photographs of vagrants and told him to rid his work of sentimentality. In 1956, in Paris, Davidson presented his photographs to his hero, Cartier-Bresson, and began "a friendship and mentoring" that lasted until the latter's death in 2004. Davidson joined the Magnum agency in 1958 and began a journey that has since made him one of the great pioneers of American documentary photography.

Outside Inside records Davidson's photographic journey chronologically, over three time frames: 1954-1961, 1961-1966 and 1966-2009. The first volume includes his groundbreaking series, Brooklyn Gang (1959), which may also lay claim to being the first visual chronicle of postwar outsider youth culture in America. It focuses on the Jokers, a gang of teenagers he spent months befriending and shooting.

The following year, he spent time on the set of The Misfits, where his wonderful snatched portrait of Marilyn Monroe and Arthur Miller was taken. Then, in 1961, he trailed the "freedom riders" of the American south during the nascent civil rights struggle, producing a body of work that stands as one of the most riveting testimonies of that turbulent time.

Later, Davidson turned his attention to what he termed "worlds within worlds", most notably in the images that make up East 100th Street (1970), an in-depth portrait of a single block in New York's East Harlem. All human life is here in its dignity and suffering, abandonment and struggle. The strange intimacy of many of the portraits caused some contemporary critics to accuse Davidson of exploitation, but today they seem ahead of their time in terms of their style and their subject matter.

"I view my work as a series," writes Davidson. "I often find myself as an outsider on the inside, discovering beauty and meaning in the most desperate of situations."