What is photography about? This is the question every photographer must ask him or herself every day - and the answers are as varied as the number of photographs in the world. And that, as we know, has increased exponentially in the years since John Szarkowski compared the number of photographs to the number of bricks; since the advent of the internet, there are far more photographs around than bricks.

Every photographer answers the question in the images they make, but not so many choose to answer it formally by making a trio of high-profile photobooks. That is what Paul Graham has done in his ‘American trilogy' - American Night (2003), a shimmer of possibility (2006) and The Present (2012).

Note that the question I have posed asks what ‘photography' is about, not photographic art or the photographic media. Photography, pure and simple. Well, hardly pure and hardly simple. Graham has consistently questioned the notion of photography as art - ‘conceptual' photography, fabricated photography, artsy-fartsy photography - not because these genres are bereft of achievement, but because they have achieved a certain cultural hegemony in the art market and museum. "There remains a sizeable part of the art world that simply does not get photography. They get artists who use photography to illustrate their ideas, installations, performances and concepts, who ‘deploy' the medium as one of a range of artistic strategies to complete their work. But photography for and of itself - photographs taken from the world as it is - are misunderstood as a collection of random observations and lucky moments, or muddled up with photojournalism, or tarred with a semi-derogatory ‘documentary' tag," he has said.

Yet Graham has also had a tilt at unthinking documentary in the past. In one sense, this trilogy is doing exactly that. "I have dissatisfaction with classic documentary language. It was wonderful when it was invented. But it has to be alive, to grow and develop, just like the spoken word. We don't speak the same way we spoke in 1938 or 1956, so why should we make pictures the same way?"

Spread from Paul Graham's The Present, the third and final part of his American trilogy.

So Graham has lambasted the pretensions of both ‘art' and ‘documentary' photography. He is suspicious of the simplicity of documentary photography and its often fake concern, its pretentious ambition to ‘change the world'. He talks about escaping the ‘tyranny of narrative', the need to tell a story. He stirs it up left, right and centre. So what is he proposing?

Firstly, he nails his colours to the mast of photography, saying: "I don't get tired of trying to understand and look at the wonderful amazing nature of what's around us." And he lays out a fairly succinct mission statement for the American trilogy: "How do we articulate this uniquely photographic creative act, and express what it amounts to in terms so that the art world, highly attuned to synthetic creation - the making of something by the artist - can appreciate serious photography that engages with the world as it is?"

His answer is to look formally at the basic vocabulary of photography, by making images that explore the photographer's basic tools, such as exposure, focus and the photographic moment. And he also hints at what photographers actually do at the moment they take a photograph. The American trilogy, on one level, is a formal treatise on the act of photographing. But if that was all it was, I would not be so interested in it.

Spread from Paul Graham's The Present, the third and final part of his American trilogy.

John Szarkowski frequently used to voice his excitement at the way Eugene Atget disposed forms within the picture frame, and rightly so. This equates to the formal aspect of Graham's project. But, as Max Kozloff pointed out (with equal rectitude): "If an Atget image were endowed with all the (formal) grace in the world, but bereft of Atget's time-honoured sense of the world, we would not recognise or care that the image is his. It would be truly soft in content, regardless of how well masses are arranged within the frame."

Fortunately, Graham has a complex sense of the world, and of the medium. Even calling it the ‘American trilogy' looks beyond the obvious. It indicates photographs made in America, but also (and this is not as simple as it sounds) that the work is about America. It signifies that Graham is a socio-political photographer, not in a tub-thumping way, but in the way that many contemporary American photographers are today, telling photographic tales about an America that, as in the 1930s, is particularly unsure of itself.

The trilogy's first part, American Night, refers to that uncertainty, and is the most overtly ‘political' of the trilogy. Photographs of large new homes, certainly not at the bottom of the market, are contrasted with images of single figures in the landscape, figures from a different economic demographic to the real estate, and also of non-Caucasian ethnicity. Frequently, these figures are printed in an extremely overexposed manner, which has the effect of ‘whiting' them out and making them more or less invisible.

It is a simple yet pertinent message that exposure can be used as a creative tool, utilised here to make the point that non-whites are largely invisible to majority-white America. 

I find American Night the least successful of the trio, not because the message is too obvious - it happens to be largely true - but because the photography is not so interesting. The project worked better for me on the wall when I saw it, but I think it was the impressive size of the prints rather than their intrinsic quality. But with that stricture, let me say two things. Firstly, one of Graham's virtues is that he never repeats himself, or retreats to a comfort zone, like so many photographers. He is constantly experimenting, and that can bring its own problems.

Secondly, the kind of photographic voice for which he is searching in the American trilogy is a high-wire act in itself. Graham is looking to distil ordinary experiences, but not, like other photographers, by doing it with obviously extraordinary images. He is trying to replicate the commonplace with ordinary photographs, making images that verge on the mundane or boring, but, as John Gossage has put it, "boring in an interesting way".

Spread from Paul Graham's The Present, the third and final part of his American trilogy.

And if he doesn't quite do it in American Night, he certainly does in a shimmer of possibility, rightly voted the best photobook of the past 25 years at last year's Paris Photo. In 10 volumes of street sequences, ranging from a single picture to more than 60, Graham details beautifully the smallest incidents of urban or suburban life: a man dragging on a cigarette, a woman eating a burger, a man mowing grass. Here, he gets the balance right between picture-making and the stories - or non-stories - he is telling; moments of everyday poetry, ‘filmic haikus' he calls them, that are only vaguely narrative in tone but catch the non-events of life and make something significant of them. He does it without drawing attention to the fact with too much poetry or sentimentality, or too much of a sense that things have been forced.

shimmer also has the best balance within the trilogy between the formal and the social. In American Night, the disregarded were rendered invisible. In a shimmer of possibility, they are made visible, without making too big a deal of it, and yet in a manner that demands our attention and our thought.

Inevitably, The Present doesn't quite match this - although it has a good try. Graham's diptychs and triptychs of closely consecutive moments on the streets of New York certainly catch the moment, emphasising how randomly our street experiences shift from second to second, sometimes quite startlingly. But this part of the trilogy works better in the gallery. When seen in the flesh as large-scale prints, The Present is absolutely magnificent. One can really get into them and inhabit them, making the sense of street experience articulated by Graham all the more vivid.

Although one can carp at certain aspects of the American trilogy, its overall ambition and achievement is not in doubt. Graham is a key figure in a new era of American ‘documentary' photography. Actually, documentary is the wrong word, I prefer to call them ‘storytellers'. Figures such as Doug Rickard, Deborah Luster, Raymond Meeks, Mark Steinmetz, Greg Halpern and Christian Patterson are using the same kind of elliptical narrative, weaving fact with fiction, exploring the medium's formal language in different ways, to talk, however obliquely, about the state of America in this anxious era. Paul Graham might be a Brit living in the Big Apple, but his American trilogy has become a key monument of contemporary American photography. Just ask Doug Rickard, Christian Patterson, Ron Jude or John Gossage.

Visit www.paulgrahamarchive.com.