Today Harrison still works with some of the world's greatest photographers but on a very different level. He gets to root around their archives, read their diaries, be told things that he can never publish ("the best book by me is the one you'll never see," he says) and decide which of their images go into their exhibitions or his books.
His latest project has been to curate the Ron Traeger part of the forthcoming Triple Exposure show at the V&A and to write the book that will accompany the exhibition. It was a poignant business: Traeger, one of the most promising 1960s photographers, died aged 31 in 1968. His widow, Tessa, was pregnant when Traeger discovered he had cancer (he died of Hodgkins's disease) and she subsequently lost the baby. Throughout the intervening years - without an exhbition of Traeger ever having taken place - she clung, Miss Havisham-like, to her husband's work. It does not take a great leap of imagination to realise that with this show, she has finally been able to give birth to something of Ron's.
"This exhibition has been very significant," says Harrison. "It's as if Tessa had been storing it all up and now she can finally let go. When the last print was made [for the V&A exhibition] she had the whole studio redecorated. It was an exorcism." Just before the paintbrushes came out, Harrison had taken Stuart Brown, Traeger's old assistant, to lunch. "We popped into Ron's old studio [now Tessa's] and Stuart said, "My God, it hasn't changed since 1968!'"
For the Traeger show, Harrison started as he always does when putting together a "project" - by going through the archives, and doing a lot of research. Once or twice a week, for six months, he would visit the studio and methodically sift through the shoe boxes that Tessa had kept, full of prints, contact sheets and transparencies. It was a little like going through a time capsule. The American-born Traeger had spent some time in Paris, and "there were these envelopes from processing labs with groovy 1960s French graphics on them." In one old box, Harrison found four frames of Twiggy wearing huge Sixties glasses (the photograph on the left). "There was nothing else like them on the rest of the film. It wasn't a picture Ron had ever picked himself. But the moment I saw it I knew I had to have it for the cover of the book. But Tessa absolutely hated it and there was a big fight for me to have it." Harrison won.
In curating a show, there are inevitably disputes along the way. With the recent Bailey show, Birth of the Cool, Harrison had "great plans" for the lay-out; he wanted the images to appear frameless - as if they were floating. But Bailey wanted black frames, and this time Harrison lost. "Bailey says: 'You know me Martin, I'm not anal about my work. Let them do what they want.' But at the crucial moment, he cares a lot."
This, says Harrison, talking of no one in particular, is what he finds most difficult about photographers. "They say they want you to curate their work, but at some stage they always want to interfere." What makes him do it? "I love making someone's work the very best it can be and I never tire of looking at pictures." What makes him think he has a right to choose the prints we see? "I think I am the best picture editor ever," he says.
Other photographers vouch for Harrison's skills. Lillian Bassman, whose work appeared in Harpers Bazaar in the 1940s and 1950s, and about whom Harrison has written a book, thinks Traeger is in good hands. "We spat a little when he went through my prints, but Martin is very astute and has great taste," she says. "He will go cross-country just to get a quote." One of the biggest names in fashion photography, Paolo Roversi, whose book Harrison is currently working on, concurs. "Martin is like a detective in discovering things about people," he says. "He really knows what he's doing."
This assuredness is a far cry from his first day as a fashion photographer's assistant, a job he got by pretending he knew more than he did. On his first day he "stood rooted to the spot until another assisted helped me". After three years, Harrison realised he was becoming a photo historian. "I would spend my spare time in the library, looking at back issues of Vogue and Harpers, analysing Richard Avedon's latest shoot, or in the basement at the Tate cataloguing whether Proserpina faced left or right."
Since then he has written more than 20 books, mostly on photography (including Appearances, a legendary tome which is now out of print), but a couple on stained glass (his other speciality) and one on Edward Coley Burne- Jones. Harrison has been in charge of the Olympus Gallery and rifled through the psyches of the world's greatest photographers. As he'll probably never write the "best book", here are a few snippets about some of our greatest photographers. Paolo Roversi: "He had a version of the World Cup made for my son to win at football last summer. That's the sort of guy he is." Richard Avedon: "The best photographer of fashion ever". Cecil Beaton: "He loved my crushed velvet trousers and would ring up Vogue requesting the boy in the nice trousers. Once I walked in on him taking a pee and he just said, 'Do come in.'" Patrick Demarchelier: "Dead easy to work with." Bruce Weber: "The photographer that wants to retain the most control."
"The rest," says Harrison tapping his temple, "is all up there".
'Triple Exposure: 3 Photographers from the 1960s. Michael Cooper, John Cowan, Ron Traeger', Canon Photography Gallery, V&A, 16 September to 30 January (0171 942 2528)