According to a list of previous books printed in On the Plain of Snakes, this is Paul Theroux’s fifty-first full-length work. Demoralising news for the sluggish, especially as the new offering is wonderful.

Theroux, now seventy-eight, sets off along the ‘rusty-ribbed border fence’ covering the whole distance from Tijuana to Matamoros, weaving between the USA and Mexico before plunging south. The titular plain of serpents is a translation of Coixtlahuaca, a place ‘in the sighing emptiness’ of Oaxaca. He covers, of course, particularly at the beginning of the book, the plight of illegal migrants, their passages facilitated (or not) by coyotes who provide inner tubes in which to cross rivers. Many die of thirst and lie unburied in the sun until their corpses disintegrate.

What does Theroux do on his trip? In Nogales he has his teeth whitened, in San Diego de la Unión he attends a first communion, in San Miguel de Allende he drops in on a wedding and in Monte Albán he inspects pyramids built at a time when ‘Britain was a land of quarrelsome Iron Age tribes painting their bellies blue and huddled in hill forts’. Sometimes he abandons his car, which has Massachusetts plates, in a secure car park and goes on bus journeys. Mexican highways are well maintained, he notes, but the off-ramp ‘always leads to the dusty antique past – to the man plowing a stony field with a burro, to the woman with a bundle on her head, to the boy herding goats, to the ranchitos, the carne asada stands, the five-hundred-year-old churches, and a tienda, selling beer and snacks, with a skinny cat asleep on the tamales’.

Theroux quotes widely from published sources in both Spanish and English, interviews officials and, as always, talks to ‘ordinary’ people, including some who barely speak Spanish (the Mexican government recognises 68 languages and 350 dialects). Allowing his interlocutors to tell their stories, Theroux deploys his usual potent blend of specificity and accumulation, his technique honed over many decades.

In two of this long book’s extended highlights, the prose relaxes. In one, Theroux teaches an adult writing course for ten days in the Roma Norte district of Mexico City. He forges firm friendships there and the reader senses his spirits rising as he gets a purchase on Mexico through these relationships. In the second sustained episode he attends Mexican Spanish classes for three weeks at the Instituto Cultural Oaxaca. The colonial city has resisted modernisation and Theroux feels at home there. It is sixty years since he left school. On the first day, he writes, ‘I clawed at my cuff and sneaked a look at my watch, assuming half an hour or so had passed. But it was only ten past nine.’ One of his fellow students is the same age as his car (thirteen).

From the start, he is alert to corruption, poverty and danger. Campesinos in some states are poorer than their counterparts in Bangladesh, ‘languishing in an air of stagnant melancholy on hillsides without topsoil’. More than once a crooked policeman stops his car, threatens him – Sabes qué te puedo hacer? (‘Do you know what I can do to you?’) – and demands hundreds of dollars. Theroux has no choice but to pay. Of one place, a waiter reassures him, ‘They’re only killing ten people a day.’ More than thirty thousand murders took place countrywide in 2017. Theroux conjures the horror of it all. Human heads are displayed at intersections; the word ‘desperation’ echoes through these pages like a knell.

At the end Theroux spends time in the mountain city of San Cristóbal de las Casas among a self-governing community of Zapatistas, a movement that had its origin in demands for dignity and justice for the indigenous people of Chiapas. One of his language school friends inveigles him into a secret conversatorio, a conference of sorts – and I won’t say more, to avoid spoiling it. This is a fabulous section of the book (which is divided into five unequal parts). The experience offers the author a redemption of sorts.

Readers who have followed Theroux through the decades will find here an old friend, albeit a crusty one. Have his preoccupations changed since his youthful self travelled through Mexico by train forty years ago, the trip recounted in The Old Patagonian Express? He refers to decrepitude quite a bit, complains less and writes about food more – perhaps small pleasures are accentuated in senescence. (He describes, gorgeously, ‘an anthology of black clotted beans’ he sees in a restaurant.) But most of all he looks more deeply into a divided country and tries to make sense of it. On the Plain of Snakes is a superb book from a master of his craft, and perhaps the author’s best yet. A few young authors are making headway in the travel genre Theroux practically reinvented in the 1970s. But they still can’t touch him.