I must declare my connection to Louis Theroux. I was in the year below him at school and university. He hasn’t changed very much. He was just as gentle then – dopey even – as he was on the telly in Weird Weekends, when he investigated the strange American sects he writes up in this book. What is less apparent on telly is his acute intelligence, telegenically veiled by that dopiness. He was so clever at Westminster that he jumped a year. And he got a First in Modern History at Magdalen.

But he never had the brittle arrogance that often goes with that sort of record. To be clever without wanting to glory in it, put dimmer people down or make an act of covering it up (viz Boris Johnson) is rare. At school and university and on the telly, he showed his intelligence in a soft, Socratic way, asking interconnected questions whose answers gradually built up until you emerged as a pretty good fool.

This is all a roundabout way of saying how beautifully suited Theroux is to television. The Socratic technique worked like a dream in his Louis Meets… series, exposing the vaingloriousness of, among others, Ann Widdecombe, Neil and Christine Hamilton, and Keith Harris. In a television world of staged aggression that lacks focus (Jeremy Paxman, everyone on the Today programme) and the staged softness of the celebrity interview (Martin Bashir, Sir Trevor McDonald), Theroux’s line in ever-so-nice character assassination is a one-off.

Theroux has come less to assassinate than to investigate in this alluring book, where he returns to America to see his old television interviewees six years after he last interviewed them – porn stars, gangster rappers, white supremacists, UFO followers, get-rich-quick gurus and anti-government fanaticists. His aim is no longer to skewer his subjects on the contradictory points of their beliefs. This time, he’s trying to establish what makes them tick, and whether their different sorts of weirdness have shared roots.

Paul Theroux’s son writes with just as clear an eye for character and place as his father. His sensitive ear picks up useful neologisms: the ‘aborticide’ of 50 million children since 1970, the horrors of the pair known as ‘Billary’ Clinton, terms coined by anti-government obsessives who tried to set up their own mini-country, ‘Almost Heaven’, in Northern Idaho in the 1990s.

And he’s funny, particularly with the Reverend Jerry Gruidl, the Nazi supremacist in Idaho, who admits to being an Are You Being Served? fan. Theroux asks Jerry to repeat the Mr Humphries catchphrase, ‘I’m free.’ Jerry refuses: ‘But I’m not free! Because this country’s in bondage to the Jews!’

Theroux is alive to the cruel comedy of the nastier sort of weirdness as it comes up against everyday suburban American life. The California home of the two sixteen-year-old sisters in a Nazi singing duet, Prussian Blue, is entirely normal, except for the two little pairs of skinhead boots by the front door. The bumper sticker on their truck reads, ‘My Boss is an Austrian Painter’. The only computer game the girls are allowed to play is one devised by white supremacists, where a skinhead goes through a ghetto shooting Mexicans and blacks who are perched on basketball hoops making gorilla noises.

Theroux’s final analysis of American weirdness is true and new. Weirdness is self-sabotaging; that’s why it’s weird. It goes against your moral, financial or social interest to have sex on camera for a living, to pay to be hypnotised into becoming a millionaire, or to prepare for an  apocalypse that never comes. Why do it then? Why don’t Theroux’s interviewees come to their senses now, years after he first talked to them on telly? In fact, it doesn’t much matter to the weirdos if something is true or false. What matters is how important it makes them feel.

We have all come across the pub conspiracy theorist who grandly tells you that MI6 killed Princess Diana, or that the Druids hummed the stones of Stonehenge from the Preselis to Salisbury Plain. The reason why the pub bores are so proud is entirely that their theories are so unlikely. A man in the pub who told you that Princess Diana died because her chauffeur was drunk, or that the Stonehenge stones were transported by conventional means, could hardly claim to be a curiosity.

Once Theroux realised that all his interviewees shared this desire to impress through fantasy, they lost some of their nastiness and edge. The white supremacists seemed more pathetic; the gangster rappers merely irresponsible, as opposed to poetic; the money-making gurus more straightforwardly manipulative than odd.

Theroux’s criticisms of the shortcomings of weird Americans are all the more convincing for his willingness to praise them. It’s that even-handed gentleness again. He’s perfectly ready to admit that Jerry Gruidl can also be pretty decent. When Theroux loses his computer, Jerry takes a stack of flyers advertising the lost computer round town and tracks it down for him.

Theroux is then presented with the conundrum: what do you give a neo-Nazi for a thank-you present? Very few people know a neo-Nazi. Even fewer know a neo-Nazi well enough to be done a favour by one. Only lovely, gentle, clever, unique Louis Theroux bothers to buy one a present.

He settles for an anti-Bush quiz book.