In the early 1960s, as the Cold War was in a phase of mutual thermonuclear menace, the American man of letters Edmund Wilson turned a skeptical eye to the values supposedly being defended by the United States. About the high ideals of the enemy, he had no illusions. Like many intellectuals of his generation, Wilson spent part of the Depression giving the Soviet Union the benefit of the doubt, but he lacked the capacity for self-deception to continue with that attitude for very long. That did not mean silencing any misgivings about his own country, however.

"In relation to our Cold War opponent," Wilson wrote, "our panicky pugnacity as we challenge him is not virtue but at bottom the irrational instinct of an active power organism in the presence of another such organism, of a sea slug of vigorous voracity in the presence of another such sea slug."

In a way, then, Tim Weiner's Legacy of Ashes: The History of the CIA is a study of the inner workings of the sea slug's nervous system. It traces the emergence of the CIA from the leftover espionage apparatus of the Second World War, and chronicles its decades-long growth - most of it free from any meaningful oversight by the government to which it was, in theory at least, accountable.

But the book covers a much longer period than the Cold War itself - and in many ways it is very much a product of the post-9/11 world, in which not only the legitimacy but the effectiveness of the CIA is very much in question. The author has written about the agency for The New York Times for many years, and won the Pulitzer Prize for his reporting on secret national security programs. He is not what anyone would call a radical. But Weiner writes with a pen dipped in the acids of suspicion that built up while covering his beat.

His driving argument is that the agency's problems are not the result of any one CIA director's influence, or political pressure from on high - or even the bewildering loss of a clear sense of mission following the Cold War. The agency's problems are structural, even genetic.

In part, they flow from the division of "the future leaders of American intelligence" into "two rival camps" 60 years ago. "One believed in the slow and patient gathering of secret intelligence through espionage," Weiner writes. "The other believed in secret warfare - taking the battle to the enemy through covert action. Espionage seeks to know the world. ... Covert action seeks to change the world."

The author's instincts as a reporter are too deeply bred for him to indulge in much sustained editorializing. But the clear implication throughout Legacy of Ashes is that the CIA was compromised from the start by the allure of covert action. Cloak-and-dagger operations not only drew resources away from the necessary work of gathering intelligence, but fostered corruption. The agency's more sinister activities were concealed, not just from elected officials, but from its own leadership.

Meanwhile, the work of gathering and analyzing information was neglected - even in the areas where, one always supposed, the CIA must surely know its business. Weiner cites an in-house document from June 1956 reporting on the work of the spies it had recruited in the Soviet Union. There were just 20 of them, and only two had any connection to the government or the military. The CIA was, in short, carefully harvesting whatever state secrets might come the way of a Russian veterinarian.

Four months after that report was prepared, a massive uprising against the Communist government took place in Hungary. The CIA had no spies in the country, and scarcely anyone at headquarters who even knew the language. The agency "had no idea that the uprising would happen, or how it flourished, or that the Soviets would crush it," writes Weiner.

Lacking hard information, the CIA often enough relied on guesswork. In 1960, its analysts confidently stated that the Soviets "would have 500 ICBMs [intercontinental ballistic missiles] ready to strike" the United States within a year. "But Moscow did not have 500 nuclear missiles pointed at the United States at the time," Weiner notes. "It had four."

And when someone at the CIA did produce an informed, long-term estimate of the world situation, it seems not to have made any difference. For a few years in the mid-1970s, "the CIA's reporting on terrorism in the Middle East was better than it ever had been," as the author puts it, "or ever would be again." During that period, the agency produced a study on "the wave of the future" that sounds rather prescient. It recognized, in the report's own language, "the development of a complex support base for transnational terrorist activity that is largely independent of - and quite resistant to control by - the state-centered international system."

But within a few years, all attention turned to the Soviets as the sponsors of most evil on the planet. By the time Ronald Reagan was in office, the agency "had next to no good sources on terrorism in the Middle East."

Not that the revitalized Cold War made the CIA any more astute about the enemy. In early December 1988, it declared in a report that "the basic elements of Soviet defense policy and practice thus far have not been changed by Gorbachev's reform campaign." Within a week, Gorbachev announced that troop levels would be reduced by half a million soldiers.

These various intelligence blunders (other examples abound) form only part of the narrative of Legacy of Ashes, which also chronicles some of the more notorious episodes in the CIA's history of covert operations - the coups, the assassinations, the torture programs; the foreign politicians whose loyalty we rented with large suitcases full of cash; the efforts to kill Fidel Castro, or at least make his beard fall out.

All of which is appalling and disgraceful, though not much of it is new. The same caveat goes for the "revelations" in the set of documents the CIA has recently released under the title "Family Jewels." By now, the CIA's longtime role as secret army ought to be known to every American, though clearly it is not. At that level, Legacy of Ashes deserves a wide readership, and will probably win one given the page-turning gusto of its narrative. (It also creates a somewhat sensational effect by showing just how many important figures in the CIA were severe alcoholics or mentally ill.)

The past is prologue. In recounting the CIA's life and crimes from one presidential administration to the next, Weiner debunks any notion that things became uniquely bad under George W. Bush. He also undermines one of the more durable myths about the agency, not just within the United States but among people around the world. That is the belief that the CIA is (or at least once was) an extremely well-organized and practically omniscient entity - one sinister in its methods, perhaps, but certainly quite efficient.

Well it was indeed sinister enough, in its prime. But no aura of superhuman power remains. It's hard not to think of the image that Edmund Wilson used, years ago. "An active power organism," he wrote, "a sea slug of vigorous voracity." That is the picture of the CIA left in the mind's eye by Legacy of Ashes.