House of Bush, House of Saud By Craig Unger

On 13 September 2001, the US imposed a nationwide no-fly zone, and yet more than 140 individuals were permitted to leave the country. Nearly all were Saudi, and roughly two dozen were kin to Osama bin Laden. Who allowed them to leave? Given that 15 of the 19 hijackers were Saudi, what was the rush in squandering what may have been an intelligence mother lode? Craig Unger first reported this story in Vanity Fair.

Now, in House of Bush, House of Saud, he places this incredible scenario in the context of a decades-old relationship between the ruling clan of Saudi Arabia and the US's pre-eminent political dynasty, the Bush family. Although the book's UK publisher has withdrawn it because of libel concerns, it has already made a big impact in the US, where it hovers on the New York Times bestseller list along with Richard Clarke's explosive memoir.

In a year when the President will campaign as tough on terror and homeland security, Unger's book is essential reading. Not only does it pose disturbing questions about Saudi involvement - witting or unwitting - in the September 11 attacks, it presents a frighteningly believable case that the Bush administration's cosy relationship with the house of Saud precipitated this catastrophe.

Unger takes us back to the 1960s, when George HW Bush was a Texas oilman drilling the first offshore well for Kuwait. Bush got out of oil in 1966 to get into politics, and wound up as head of the CIA just as Saudi businessmen close to the country's royal family began investing in Bush's home state. They bought real estate and purchased planes. They bought a bank in Houston with former Texas governor John Connally.

During the 1980s, as petro-dollars flowed into Saudi Arabia, the country became a convenient money and weapons launderer in US attempts to stop extremism. The US sent money to Nicaragua through Saudi Arabia, in exchange for arms. During the Iran-Iraq war, the US supported Saddam Hussein - whom the CIA had first hired as a 22-year-old assassin in 1959 - by giving him guns and bombs through Saudi Arabia.

What emerges from Craig Unger's narrative is a portrait of how the elder Bush helped to develop a way of doing business with the world's worst thugs, which was transplanted to other regions with staggering naivety. The US alliance with the Saudis proved so convenient that it was used again to prop up the mujahedeen in Afghanistan. The Saudis matched US support dollar for dollar, and a scion of the family closest to the rulers was sent to fight and to build roads: Osama bin Laden.

The Saudis began to repay that favour in the 1980s. When oil prices were dropping, a Saudi investor close to the royal family bailed out a tiny Texas oil company, Harken Energy, when one of its directors was George W Bush. Then the senior Bush left office and began delivering speeches for the Carlyle Group. This investment firm used its contacts in government to buy defence contractors on the cheap and sell them at high profit. In this way, Unger argues, $1.4bn flowed from the House of Saud to the Bush family and their interests.

Did this relationship cloud the Bush administration's ability to detect September 11? Unger makes a compelling case that it kept American eyes off rising extremism in Saudi Arabia. Even after the attacks, Saudi officials refused to freeze bank accounts; they refused to allow Americans to use their soil during the invasion of Afghanistan; finally, there is even some evidence that someone close to the Saudi royal family knew that an attack was coming on 11 September.

House of Bush, House of Saud will be labelled conspiracy theory, but Unger's research is too cautious to support that claim. With great care, he has synthesised scattered reports into a narrative as chilling as it is gripping. The book builds a momentum of discovery that makes it impossible to stop reading. Will the American people carry these concerns into the election? Only time will tell.