When the National Socialists came to power in 1933, the Viennese writer Karl Kraus tried to address the political and moral catastrophe, but could only find words of cosmic despair: "Mir fallt zu Hitler nichts ein" ("When I think of Hitler, nothing occurs to me", or "Hitler leaves me lost for words"). Ever since, generations of writers have been trying to make up for Kraus. Everything occurs to us about Hitler; so far from "lost for words", hundreds of millions have been written about him and his colleagues and what they did to Europe. These pointless people and their reign of empty destruction seem to be our dominant obsession more than half a century after the thousand-year Reich disappeared in the ashes.

Among Adolf Hitler's weird court of mountebanks and madmen, Albert Speer exercises a peculiar fascination, because he wasn't an obvious scoundrel or cheap crook like the others. He was an intelligent, educated man of considerable gifts, and, as Joachim Fest shows, the only one of the gang who could contradict Hitler or stand up to him. Speer had a cosseted upbringing in Mannheim, where his father was an architect. He was too young for the Great War and avoided the postwar hyperinflation because his father had shrewdly sold the family properties for dollar treasury bonds. After studying architecture himself, Speer spent several years at a loose end, and he was one of the many superfluous men to whom Hitler's rhetoric spoke. In March 1931, he joined the Nazi Party and, in 1933 he was summoned to lunch - "I thought I'd faint" - by the new leader. He designed a stadium for a Nazi rally in Berlin, and became in effect the movement's art director.

His latest biographer is one of the most eminent writers on the Third Reich. When Fest edited Speer's own memoirs, he accumulated quantities of notes. He knows the subject backwards, and he writes with authority (though his book is not helped by a sometimes clunking translation: Goebbels "prided himself on his barbaric dash"; Hitler's war councils saw a "combination of omnipotence mania . . . and intransigency"; and the "more than 4,000 British and American fleets" that dropped bombs in one day must mean aircraft).

His first wartime job was building armaments factories; Speer became minister for munitions in 1942. By the following year, he was responsible for the entire German war economy. He was without doubt an administrator of genius who did the job with remarkable success and, despite everything, German war production increased steadily until the summer of 1944. All this happened amid the regime's revolting atmosphere of intrigue and back- stabbing, which Fest describes chillingly. On one vexed question, he makes a very important point. Speer has sometimes been cited by those who would justify the RAF's campaign of terror bombing, and he certainly testified to the sheer destruction of Germany in the last year of the war, when the people were barely surviving - though still fighting - amid the rubble. Plainly, incinerating all of Germany's cities and killing hundreds of thousands of civilians could not have helped the country's war effort, but the evidence goes against any claim that Bomber Command made a decisive contribution to winning the war.

Quite apart from moral considerations, Arthur Harris's obsession with flattening as many cities and killing as many people as possible was plainly a mistake in military terms. His refusal to pursue the selective bombing of vital war industries "was both costly and nonsensical", Fest writes, and "considerably prolonged the war". The Dambuster raid of May 1943 destroyed or badly damaged some of the dams supplying the Ruhr, but Harris, never liking such "panaceas", did not press on by attacking the neighbouring dams, the effect of which would have been to bring the Ruhr to a standstill. It was the same with the brilliantly successful American raid on the Schweinfurt ball-bearing plant the following August. If there had been an immediate second strike, which Harris obstructed, armaments production would have collapsed and the war might have ended that year. Speer was astonished by this negligence and spoke with irony of his powerful ally among the British general staff.

At any rate, amid the horrors of the last months of the war, Speer claimed to have refused to obey Hitler's orders to lay waste the whole of Germany. But the crucial point in Speer's life was not then, but the following year at the Nuremberg trials, when he escaped the gallows. After his startling acquittal in the "Happy Valley" murder case in wartime Kenya, Sir Jock Delves Broughton received a telegram, which was later hung in the bar of White's to amuse the racing chaps: "Congratulations, understand you a neck cleverly." But no domestic murderer ever won a neck more cleverly than Albert Speer. To this day, it is hard to say which is more astonishing: his audacity, or that he got away with it. Apart from skilfully playing the Anglo-Americans off against the Russians, his strategy was to "cop a plea", recognising the unparalleled crimes of the National Socialist regime and accepting moral responsibility for them in the abstract, while insisting that he had been ignorant of the worst crimes, above all, the extermination of Jews. This mixture of apparent penitence and apparent innocence worked a treat, and he was sentenced to 20 years in prison while other, less adroit, defendants were hanged. At his trial, Speer was, as Fest writes, "still expecting a significant postwar career", and indeed he found one, emerging from Spandau for 15 profitable years as an author and chat-show guest, before dying on a visit to London in 1981.

Not everyone fell for the Speer spiel. One of the Americans interrogating him, the economist J K Galbraith, was sure he had concocted a cunning ruse. Decades later, Speer was interrogated again, this time by Gitta Sereny, who peeled away the onion skins of memory until she had him begging for mercy, and showed, to her own satisfaction, that he had known far more than he wished to acknowledge. All of this is carefully described by Fest, who takes it at face value - as indeed he has a tendency to take the whole ludicrous National Socialist movement at its own estimation. Looking at the way Speer designed the Nuremberg rallies with the latest technology to produce "an atmosphere of magic, mystery and exaltation", Fest writes that many visitors were so impressed by the spectacle "that they were prepared to overlook some of the regime's more repulsive features". But those displays of deranged kitsch were some of its more repulsive features, as were Speer's plans for rebuilding Berlin as "Germania". He was visited there by his father, about the only attractive figure in this book, who thought Hitler was a "criminal upstart" and who said, after looking at his son's grandiose schemes, "You've all gone completely mad."

"All" was the operative word, for a whole people and a culture. It's not just that Speer was obviously lying. How could anyone have ever seriously believed that one of the most senior figures in the Reich was ignorant of its single greatest achievement, the industrial killing of millions of people? It barely matters whether or not he was present at the meeting at Posen in October 1943, when Himmler said that "the difficult decision had to be taken to make this people vanish from the earth", but that it was done without "our men and our leaders suffering injury to spirit or soul". Even if he wasn't there (and he probably was), he might as well have been.

As Fest observes, Speer conveniently looked away to avoid seeing things he didn't want to see, but he also provides more evidence of his culpability. There is a subplot about the Chronik, the Resettlement Department Chronicle compiled by Speer's colleague Rudolf Wolters, which made it clear that Speer was well aware of what was happening to the Jews of Berlin. As Speer, freed from prison, acted out his role as a man duped, Wolters berated him for his treachery to the good old cause, in effect using the Chronik to threaten him with blackmail, and it is almost possible to sympathise with Wolters.

Yes, of course Speer knew. But then, all the Germans knew: not every detail about railway timetables and gas chambers, but about the broader fate of the Jews. A whole people tried to look away, so as to avoid injury to spirit or soul. Speer's "I acknowledge the crime of which I was sadly unaware" was, in a sense, the plea of the Germans as a nation - and one that, paradoxically, the Nuremberg trials made easier.

During the Auschwitz trials of the 1960s, the German novelist Martin Walser noticed that every horrible revelation made the Germans feel more distant from the accused, and thus less culpable themselves. Similarly, those hanged at Nuremberg were - like Hitler, Himmler and Goebbels with their suicides - truly sacrificial offerings on whom the sins of a people could be loaded.

As Hugh Trevor-Roper has said, Speer was "the true criminal of Nazi Germany", which means, above all, that he was an extreme case of a vaster criminality. As Ian Buruma has said, in Hitler's Germany, Speer was not abnormal at all, but just an ambitious technocrat doing his job as well as he could, "an opportunist and a conformist" like most of his countrymen. And as Joachim Fest now writes, Speer was a man without any deep ideology, who had a public and a private world, "and the two were kept strictly apart". I wonder if Fest realises how much this makes Albert Speer the supreme German of the 20th century.

Geoffrey Wheatcroft is the author of Controversy of Zion