The American edition of Naked at Lunch has the title in big upper-case letters, printed as though they were cutouts, windows onto the scene behind, showing a man on a slatted chair that appears to be on a ship. You can see the sea, and the kind of light you get at sea, which has inspired artists for…

Okay. I admit it. I could avoid the naked truth forever, so let’s get it over with: the guy on the chair is NAKED, alright? NUDE. Plump, middle-aged, bald, grey goatee, specs and he’s IN THE BUFF. On his lap is a MacBook Air. It’s resting on his willy. His todger, for God’s sake, his wang, doodle, schlong, his PENIS is TOUCHING his COMPUTER. This must be the author on the nudist cruise, with 1,865 other naked people, and I really hope the ID tag around his neck doesn’t say ‘Access All Areas’.

So that’s the naked author, with his whacker and his Mac, and this is his book about nudists and what they’re like and what the hell they think they’re doing. So, not unreasonably, the book is categorised as Social Science. In the USA.

But not here. Here in Britain, there’s no nude author. The cover is whimsical, cartoony: there are little pink blobby people, sunbeds, a swimming pool and a very tanned woman with a poodle and a tent. And here in Britain, the category is Travel Writing.

So of course there’s a tent. A tent says travel, a bit of a break. A tent says liminal, that nudity is recreational and that normal service – clothes – will be resumed shortly. A tent says healthy, fruit juice, organic woodland folk, tech sandals, hairy (if female) or hairless (if male) legs and no sex. Not ever. Maybe Barbara Windsor playing nude volleyball, Sid James leering from the steps of a caravan, but that’s your lot. That’s how we do it here.

We should perhaps try to distinguish naked from nude. Naked is what you are when you haven’t got your clothes on. Nude is what you are when you have, with intent, taken your clothes off. Nakedness is a state; nudity is an upside-down metonym. It invites the very sexual speculation it claims to obliterate. It also implies the presence of a witness; and perhaps the transgression of public nudism is that there are multiple witnesses whose consent has not been sought.

I saw this in action when I was about eight. We went for a day to the nudist colony on Le Levant, one of the Iles d’Hyères. It was nice to be in the warm Mediterranean without wet fabric clinging to my little legs. My father, a doctor, was happy surrounded by nude (or, to him, just naked) people for him to inspect for defects or warning signs. My mother, on the other hand, was British. She would not remove her bathing costume, but reclined in the shelter of the rocks among the smell of immortelle sauvage and salty wild thyme. All day a steady stream of naked men diverted oh-so-casually to pass close by her. My father said this was because she was the only clothed woman in Le Levant, so they were coming to enjoy imagining what she’d look like with her kit off, a primal pleasure otherwise denied them.

Mark Haskell Smith is no nudist or naturist, or devotee of Freikörperkultur, nor is he interested in screwing other people’s wives, which, anyway, the serious nudists all deny ever happens. But it does. I once did a radio programme on a nudist club alarmingly called Eureka in the alarmingly named Pennis Wood near the alarmingly named village of Fawkham. Everyone was very nice, if rather naked and respectable. And then they showed me pictures of their party nights, when they ‘let their hair down’. This, of course, involved putting stuff on: leather thongs, stockings, cat masks, latex microskirts and all the rest – and you can imagine what all the rest was. I thought it was marvellous. Haskell Smith wouldn’t have judged.

Which is his strength and his weakness. There are diverting wonders here. Nudists all seem obsessed with writing manifestos. Charles Richter, inventor of the earthquake-measuring scale, was a nudist. The guy who spearheaded the stamping out of nudist beaches round San Francisco is called Wiener. Haskell Smith goes on a nudist cruise with 3,732 naked buttocks on one boat. He goes to Palm Springs and the majestically depraved Cap d’Agde in France, not to mention Heliopolis on Le Levant. He goes to Florida – old people, sun – to find the American Nudist Research Library. At one point he finds himself in Vera Playa, Spain, in Frankie’s, a bar festooned with nudists and run by a British expat called Alan. You can picture the rest.

And the original nudists themselves: what a bunch. Take Englishman Edward Carpenter, a 19th-century gay sandal-wearing vegetarian, friend of Walt Whitman, Isadora Duncan and D H Lawrence. Or the dreadful German Richard Ungewitter, who said nudism would help eugenics because you could tell if someone was really blond.

But Haskell Smith’s slightly jokey narrative of harmless cranks partly disguises the bewildering complexity of our relationship with our bodies, our clothes, and others’ bodies and their clothes. People are killed for wearing the ‘wrong’ clothes, and not just by religious gulls drenched in lies and ignorance: think of Sophie Lancaster, beaten to death in a park in 2007 for dressing as a goth. People ridicule and often loathe the Hasidim for dressing as if they were in 18th-century Latvia. There may well be reasons to consider them schlemiels, bullies and trombeniks, but their clothes aren’t a valid one. But if your dress code breaks mine, then put ’em up, pal. It’s nearly universal. Clothes are crucial.

Walk through central London: so many clothes shops; so many clothes magazines; so many uniforms and ritual outfits. The very polity is textile-sensitive: witness the slit-mouthed viper gawp of Wolfgang Schäuble on seeing Yanis Varoufakis without a tie.

You can hear Haskell Smith repressing the odd snigger, or, more often, pre-empting it with such reasonableness that I longed for him, at the end, either to throw all his clothes away and run giggling madly into the woods or to denounce the nudists as lunatics and exhibitionists who need to find something to occupy their minds. But that’s not his way. He looks, he reports, he blushes from time to time; mostly, he worries about sunburn and skin cancer. The book is, in part, a hymn to Factor 50. But the subject’s a tough nut. Where once our ancestors were covered in woad, we are covered in confusion. Is documenting it social science or travel writing? Certainly, I’d love to read the ethnology (and indeed the psychology) of nudism, but for now Naked at Lunch is as good as you’ll get.