If there were a biopic called The Magnum Story, about the international photographic agency that celebrated its 60th anniversary last year, it would open on a tram travelling down the Boulevard Montparnasse in 1933. Two young men, one French, one Polish, are strap-hanging next to one another, though they are obviously strangers. Later, they meet at a gathering of left-wing artists and writers and strike up a conversation. The Pole, who is a student at the Sorbonne, notices something hanging round the Frenchman's neck. It's a viewfinder, the Frenchman tells him, for a Leica. From this chance meeting between Henri Cartier-Bresson and David Seymour, and the friendship that develops between them and an exiled Hungarian photographer called André Friedmann, comes the idea for a photographic agency run by and for photographers. They will share their earnings, retain the rights to all their pictures, and decide where they go to photograph, and why.  But before the idea could be realised, war broke out, first in Spain, where Seymour and Friedmann (now working under the byline Robert Capa), photographed at the frontline and Cartier-Bresson filmed a documentary about military hospitals; then in Europe, where Capa and Seymour joined the allied forces and Cartier-Bresson spent three years as a German PoW. He escaped in 1943, the year Capa met George Rodger, an English photographer working for Life, who would become the fourth founder member of Magnum (a fifth, Bill Vandivert, left after a short time). It was another three years before the name Magnum Photos was officially registered in New York in May 1947. The story that the photographers then "divided up the world" between them always sounds grandiose, as if they'd held their own private Yalta, but it was a practical solution to the remit they had given themselves: to be where stories might happen, rather than wait to be sent.  Although the idea preceded it, the war made Magnum the agency it was. It gave its photographers a moral purpose and made their plan to document the world in pictures part of a general mission for peace. It proved that photographs had a market. Picture magazines had been hugely popular during the war and after it their readers were hungry for reports from countries (and conflicts) they might never see. In personal terms, the war was the reason Cartier-Bresson remained a photographer, rather than return to his first love, which was painting. And it was photographing the horrors of Bergen-Belsen in 1945 that made George Rodger turn to Africa and swear never to photograph war again.  That Magnum survived is something of a miracle, not only because the idea of a business run on the basis of its members putting back 40 per cent of their earnings to support each other is almost unheard of these days, but also because a co-operative made up of people who essentially prefer to work alone and so might find consensus (and bureaucracy) a challenging prospect, is an unlikely recipe for success. Survive it has done, though, and grown, despite the gradual erosion of its profitable relationship with the printed press by television, and the undermining of its journalistic purpose by the art market.  From the 1980s onwards, like with other global companies, one of its strategies for survival, along with capitalising on its archive sales and transferring them to the internet, has been the reinforcement of its brand. And part of that strategy has included the periodic publication of a Magnum anthology. The latest, Magnum Magnum, a huge tome reflecting the gargantuan tendency in photo book publishing, appeared last November, presenting 69 sets of pictures, each selected by one member from another member's work. This week, the "compact" edition is published, two-thirds the size of the original, but with the same number of photographs, at a more manageable £35 rather than £95.  Members were limited to a maximum of six pictures each, which means that single pictures have often been extracted from what was originally meant as a narrative essay. Nevertheless it's a useful way of mixing old and new photographers, and it shows, to some degree, how the manner of taking pictures has changed. Apart from the obvious shift, over time, from black and white to colour, Magnum has added new members familiar with the art market (this in itself is a bone of contention for some of the older members) and their photographs have been shaped accordingly, though as Gerry Badger writes in his introduction, "Photographers do not join Magnum primarily to make art. They join because they want to make photographs about the world."  So one can identify a shift towards photographing the aftermath of conflict, rather than the action; towards composed tableaux more similar to the staged pictures favoured by the art world. But there are still plenty of photographers in the middle of a war: Thomas Dworzak in Grozny, Afghanistan and the Lebanon; Paolo Pellegrin in the Middle East, Sudan and Iraq. Despite the hopes of the agency's founders, wars are all too available to the photographer who wants to find them.  One of Magnum's strengths has been its photographers' long-term relationships with people and places they have followed for years at a time: Bruce Davidson spent the late 1960s photographing life on a single block in Spanish Harlem for his book East 100th Street; Jim Goldberg is currently photographing migration and trafficking in Europe. Philip Jones Griffiths, who died earlier this year, produced his book about the long-term effects of Agent Orange in 2004, nearly 40 years since he made his first pictures in Vietnam in 1966.  One relatively recent picture that stands out is by the German photographer Thomas Hoepker. It is a colour picture, more a snapshot really, of five friends meeting by the side of a river. One has his bike, they're all in T-shirts and sneakers, it's a beautiful sunny day and the sky is clear and blue. Across the river, as if the backdrop has been spliced in from another image altogether, is the Manhattan skyline, from which a cloud of dark-grey smoke is billowing up into the blue. This is something that a photograph can do. It can capture a dichotomy; that sickening understanding that before a bomb, or a mine, or an attack from sky, everything seems ordinary, and we're safe, then it isn't and we aren't.  As ever with these collections, it's hard to ignore the classics: Cornell Capa's Marilyn Monroe on the set of The Misfits, Leonard Freed's picture of black women clutching Martin Luther King's hand on the back of his open car, René Burri's São Paolo rooftop. But, despite its heft, this book can only provide a taster of what is now available on the web, and I wonder how long the printed anthology will continue to serve its purpose.   Magnum Magnum, ed. Brigitte Lardinois, published (Oct 6) by Thames & Hudson, £35. Images from the book will be on sale at a Magnum Foundation charity auction on October 9 2008.
Continuing her series on the best new photography books, Liz Jobey looks at the latest Magnum anthology, spanning six decades of the greatest images in photojournalism