Operation Mincemeat, By Ben Macintyre

Espionage attracts strange minds, and nowhere more so than in the macabre Operation Mincemeat, when a corpse bearing secret documents designed to mislead the Germans was floated ashore to the Spanish coast in April 1943. The plant convinced Nazi espionage chiefs, and Hitler himself, into believing that the Allies planned to invade Greece and Sardinia, instead of the obvious target of Sicily.

The story was told in 1953 by one of the two main instigators of the deception, Ewen Montagu, in The Man Who Never Was, which became a successful film. But much remained unwritten. Ben Macintyre, in his impeccably researched and highly readable book, has uncovered fascinating and recently available material, which Montagu had had to suppress.

There was the question of whose body was created as "Royal Marines Major William Martin", travelling as courier to the Allied Forces headquarters in Algiers. To appear more real, "Major Martin" was given a private life, with letters from a fiancée "Pam", whose photo was supplied by one of the secretaries with whom Montagu and his colleague, the RAF officer Charles Cholmondeley, worked in Naval Intelligence's windowless basement beneath Whitehall. Among other character-building blocks were theatre ticket stubs, a bill for an engagement ring, and a letter from an exasperated father suggesting lunch at the Carlton Grill to sort out his son's financial affairs. Naval Intelligence included three novelists, one of them Ian Fleming.

The body designated as Major Martin, far from having lived in a world of Wiltshire manor houses and gentlemen's clubs, was that of an itinerant Welsh labourer, Glyndwr Michael, found dying from ingesting rat poison in an abandoned warehouse near King's Cross. Illegitimate, orphaned, mentally ill, he was without money, friends or family. The St Pancras coroner, tipped off about the need for a corpse that wouldn't be missed, alerted Montagu. The body was removed, illegally, and stored in a mortuary until such time as it could be dumped by submarine off the Spanish coast near Huelva.

Montagu glossed over the acquisition and several flaws in the deception that could have exposed the document-carrying corpse as a plant. The Home Office senior pathologist Sir Bernard Spilsbury – memorably described by Macintyre as a "lizard in a lab coat" – had asserted there was no pathologist in Spain who could tell the difference between death by rat poison and one by drowning. Not so, as my father Ian Colvin found out when researching the case a year before the publication of The Man Who Never Was. As described in his own book, The Unknown Courier, a German police pathologist frogman based in Spain at the time specialised in deaths by drowning. He would have detected the difference between a three-month old decomposed victim of poisoning and one that had been drowned for less than a week, had he troubled to make the journey from Madrid.

What ensured the success of Operation Mincemeat was the willingness of the Germans to believe. Macintyre pinpoints the state of mind among those who should have been more circumspect. The on-the-ground agents in Huelva were looking for a coup to advance their careers. Higher up the intelligence network were people such as Hitler's chief intelligence analyst, Baron Alexis von Roenne, an anti-Nazi conspirator who may have considered the whole business smelt but saw it as a way of hastening Hitler's downfall. Once the Führer believed, none of the yes men around him dared hint otherwise. Even the sceptical Dr Goebbels recorded his doubts only in his diary.

Wartime espionage stretched the ingenuity of its practitioners, and one of the delights of this book is entering the corkscrew minds that inspired the world of 007. The gadgets expert Charles Fraser-Smith ("Q" in the James Bond films) devised garlic-flavoured chocolate for agents parachuting into France in order that their breath should smell appropriately Gallic. It was the sort of gloriously inventive touch that must have contributed to the joy that ran like a glistening thread through times far darker than today.

'The Unknown Courier' by Ian Colvin is to be re-published by Faber Finds