Books on elections are a whole different art form in the US. The genre seems to have begun with Norman Mailer's Miami and the Siege of Chicago, which turned the 1968 election into something halfway between a historical account and a novel, but which was trumped four years later by Hunter S Thompson's Fear and Loathing on the Campaign Trail.

This year's contribution comes from two seasoned reporters, John Heilemann and Mark Halperin, who have figuratively climbed into the bedrooms of the 2008 contenders to produce an account that is great fun to read even if you are left wondering about its depth and accuracy.

This is real fly-on-the-wall journalism, where long conversations are reproduced in direct speech, and the authors claim to know not just what people said, but what they were thinking.

For instance, there is an account of Bill Clinton phoning Ted Kennedy in the hope of persuading him to endorse Hillary. Kennedy was im-pressed by the relative upstart, Barack Obama. Clinton could not believe that the Democrats' senior statesman could want to back someone so raw. "A few years ago, this guy would have been getting us coffee," Clinton exclaimed.

Or did he? After this reported comment had buzzed around the blogosphere, the authors were tackled about it on Fox TV, and had to admit they did not know exactly what Clinton said. According to second- or third-hand sources it was "something just like that".

That is one drawback with this style of writing. Another is the almost complete absence of any reference to the political questions of the day. Iraq crops up, and there is a passing reference to Sarah Palin's belief that women who have become pregnant through rape should be compelled by law to see their pregnancies to term. Healthcare, the dominant issue of Obama's first year in office, does not feature in the index.

What the book offers is a wealth of insight into the incredible pressures that candidates and their staff have to endure, into the minds of people prepared to put themselves through this experience, and into their marriages – dysfunctional in every case, with the notable exception of Barack and Michelle Obama, who seem to negotiate their way through marital strife like the trained lawyers they both are.

The high comedy is saved until the end, when John McCain's team, seeing Obama motoring ahead, decided that they had to do something dramatic. With time running out, they took a huge gamble by selecting the unknown Sarah Palin, after a cursory check on her background.

Her first appearance was a sensation, and the rest of her time on the campaign trail oscillated between disaster and near-disaster. McCain's people feared at one point that she was mentally unstable.

When Obama was on the verge of entering the race for the Democrat nomination, one of his advisers, David Axelrod, warned him that he faced a formidable opponent in Hillary Clinton.

"She has to be president. She wants to be; she needs it. I don't sense that in you," he is reported as saying. The authors add: "Obama didn't really sense it, either. But he rejected the notion that running for president was a task suited only to the borderline mentally ill."

By the time you get to the end of the book, you wonder.

Race of a Lifetime By Mark Halperin & John Heilemann, Viking £25 (464pp)