How do tobacco company executives describe what they do at parties? "Oh, I'm in manufacturing/ business/ agriculture"; not, I imagine "I'm in dope peddling/ cancer creation/ legal double-dealing". Like porn film- makers and arms exporters, tobacco employees have become social pariahs. This book explains why.

At its heart is the story of Merrell Williams, the $9-an hour legal clerk who in 1994 stole thousands of incriminating company files from the British-owned tobacco company Brown and Williamson of Louisville, Kentucky. When the history of the 20th century is written, Merrell Williams may rank among those who have done most to improve the health of the world.

Like John Snow, who removed the handle from the Broad Street pump in 1855 and helped halt the London cholera epidemic spread through drinking water, Williams by his actions may have done more to halt the epidemic of disease caused by smoking than a trunkful of epidemiological studies.

From 1987 to 1992, when he worked sorting archive documents at Brown and Williamson, Williams discovered that the tobacco companies had done extensive research on the effects of nicotine and the cancerous agents in tobacco smoke, but had not published their findings. Their lawyers had set up a secret fund to sponsor and monitor "helpful" tobacco research and had suggested ways that damaging scientific reports could be censored. It was evidence of a widespread cover-up of the harmful effects of smoking - and it was all there in the documents.

Williams started to steal them, making photocopies in his lunch hour and walking out of the building with them stuffed inside his shirt. They crackled as he walked, so he would carry an open bag of crisps and munch them as he passed the guard. Stealing them became an obsession. On his last day, in February 1992, he brazenly took out his biggest batch in a bankers' box. Neither his supervisor in the documents room, nor the guard, asked to look inside.

When, two years later, the documents were made public, their effect was seismic. An industry that had until then seemed impregnable was under siege from personal liability lawyers seeking millions of dollars in damages for lung cancer victims to whom the tobacco companies had knowingly, as the documents made clear, caused mortal harm.

As the court cases have progressed, the defences of the tobacco companies have started to crumble. They are now proposing to settle for $368bn, payable over 25 years, in return for immunity. More important, they have been exposed not merely as dealers in a lethal product but as calculating, deceitful, cold-hearted killers.

It is a gripping tale - never more so than in the chapter on Merrell Williams - meticulously researched and told with pace and grace, as you would expect from a former Independent staffer. But it has one drawback. It is only half the story. So much has changed as a result of the tobacco wars of the last five years that it is inconceivable that the landscape charted in this book will look the same in five years' time.

It is not only the tobacco companies that are crumbling. Public attitudes to smoking have hardened, and governments around the world are moving against the industry. Smoking itself is under siege, and if this battle can be won, Merrell Williams will deserve his place in history.