HERE'S the first tip. If you are going to commit a crime, don't leave your specs behind. Your prescription, it turns out, can be absolutely unique, as revealing as any fingerprint or sample of DNA. That's what Graeme Pearson discovered more than 20 years ago. Then a young policeman, he was mercilessly mocked for ringing around almost every optician in Glasgow after a burglar dropped his glasses after a city-centre break-in. But it worked: the thief, a respected businessman, was caught when he ordered replacement eyewear. And Pearson, later to become Scotland's most senior detective, had single-handedly invented a new science of forensic ophthalmology.

Pearson, his new memoirs reveal, has always been something of an innovator. His book, The Enforcer, matter of factly and pleasantly strolls through a beat of nearly 40 years, one of the most remarkable careers in modern policing. It was Pearson, after all, who as a humble chief inspector in Airdrie pioneered town-centre CCTV, now ubiquitous on every street corner in Britain. As a young detective, Pearson was on hand in many of the big cases of the Seventies, often clutching a service-issue Smith and Wesson. He grappled with triple murderer Jim Harkins, helped recapture serial killer Robert Mone, and played a huge part in the case against the notorious XYY gang of armed robbers.

But The Enforcer is more than a book of "war stories" from a grizzled veteran of the streets. This is a thoughtful book that asks questions about what makes a good cop and what makes a good crook. Pearson comes to the conclusion that many of the same qualities apply to both ways of life. He writes: "Criminals, like a good detective, seek to be invisible when out and about on the streets."

Pearson, however, can no longer walk the streets of his native Glasgow incognito. His profile, as the retired director of the Scottish Crime and Drug Enforcement Agency, is just too high. Well-known criminals have been approaching him in recent weeks, quietly asking what secret he has disclosed in his book. One was an entrepreneur who once lost his specs.