Alistair Cooke was the BBC's voice of America, and his measured, magisterial tones entertained millions around the world as he reported on great events, minutiae, comic incidents and tragedies that befell the US week by week. He was born in Britain, but took American citizenship and as such was perfectly placed to construe the New World to the Old.

Shortly before his death in 2004, his personal secretary unearthed a manuscript Cooke thought had been lost - an account of his odyssey around the US in the wake of the Japanese attack on Pearl Harbour in 1941, which propelled America into the second world war.

Now published as American Journey - with a prescient foreword by another British-born immigrant journalist, Harold Evans, and the text of a 1943 broadcast for the BBC's Home and Empire Services - Cooke's travels cast an intriguing light on a country in metamorphosis. For much of the 1930s, the US had been mired in the Depression. Entry to the war transformed it economically, socially and spiritually, and led to its current pre-eminence as a global superpower. Cooke was on hand to report its genesis.

When war broke out in 1939, large swathes of the US were fervently isolationist. Many Americans traced their roots to Germany or Japan and even those without blood ties to the Axis powers saw no reason to join the global conflict. Cooke sought out the German and Japanese Americans (many of the latter were confined in harshly primitive internment camps) posing the right questions and catching the spirit of a country in flux. It was Cooke's gift not only to be in the right place in the right time, but to draw the most sensible conclusions. American Journey starts with an eye-witness description of Franklin D. Roosevelt's request to Congress to declare war. Cooke sees the ageing, crippled president limp into the House of Representatives and declares: 'For a moment we had seen him as the hurt psyche of a nation.' Four years on, a victorious US has dropped the first atomic bomb and the world has been transformed. Cooke writes: 'Man had now attained the knowledge for his own split-second destruction.'

In between the twin devastations of Pearl Harbour and Hiroshima, Cooke roamed far and wide in his quest to discover what made a newly bellicose US tick. As he set out on his trip, disingenuously bemoaning 'my own trade and its awful pretensions', he decided that 'my chief interest would be in the people'. It's the characters he comes across and their pithy, singular utterances that transmit the flavour of those times. Cooke routinely refers to 'Negroes' - as they were called - but is obviously concerned at their status as second-class citizens. A vignette of an exhausted soldier falling asleep on the lap of an obliging female stranger at a railway station says more than any official war history. And the Wyoming cattle rancher who snapped, 'Don't you forget it, son, folks around here are more dubious about Washington than the outcome of the war', would find many adherents even today.

The US that Cooke describes was beset by previously unimagined shortages, with everything from petrol to manpower absorbed by the burgeoning armed forces. Vast tracts of virgin land were being turned into military sites and newly recruited workers were pouring into small towns in search of work, camping when they were unable to find housing and, in some cases, spending their newly found wealth like water.

However, if one man can be said to represent the face of the US in the formative stages of the 20th century's greatest conflict, it was Henry Kaiser. He revolutionised the ship-building industry, turning it into a production line, while referring to a ship's bow as 'the front end'. Although his methods initially drew scorn from the professionals, in time he was turning out one ship a day, as opposed to every five months. Cooke's comment is characteristic of his sage reportage: 'America ... learned to worship, and the world to admire, this heavy man with the bald head and the expression of an amiable bullfrog'.

American Journey's style is ponderous at times, and dated by modern standards - Cooke appears a little too interested in the fate and morals of teenage girls he observes drinking in nightclubs. But, in general, his perspicacious prose shines and this is a classic document of the US transforming itself.

When Cooke died, the English language lost one of its great writers and broadcasters. American Journey is a serendipitous memento of a vast talent that has yet to be replaced.

Alistair Cooke's American Journey is released on Thursday