Stephanie Elizondo Griest is the author of two travel memoirs about Mexico: “Mexican Enough: My Life Between the Borderlines” and “All the Agents and Saints: Dispatches From the U.S. Borderlands.”

Taking a road trip through Mexico wasn’t always a radical act. Throughout the past century, U.S. artists ventured south seeking muses; surfers, waves; vagabonds, Mayan ruins; Chicanas, ancestral connections. Until the mid-1990s, my parents thought nothing of driving to Monterrey from Corpus Christi, Tex., to visit family. (It’s 100 miles closer than Dallas.) I myself spent much of 2005 and 2006 traipsing around Mexico by bus, relishing the free ham-and-jalapeno sandwiches they served with a Jumex and a smile.

But Americans’ insatiable hunger for drugs — and Mexican drug lords’ race to supply them — has since turned a luminous nation into a battlefield. Nearly 300,000 Mexicans have vanished or been killed since December 2006, when then-president Felipe Calderón tried to dismantle the cartels. A decade later, the United States elected a president whose rallying cry was “Build that wall,” despite the ineffectiveness of the 670 miles that already existed.

So when Paul Theroux left his Cape Cod home for the southern border in early 2017 in a Nissan Murano with license plates lettered “Massachusetts — The Spirit of America,” just about everyone echoed the warning issued first by a man on a Harley: “Don’t go thar! You’ll dah!” This only fueled Theroux’s mission to show that Americans and Mexicans are “at two ends of the same road.” The resulting travelogue, “On the Plain of Snakes,” chronicles seven month-long trips that spanned the length of the border and as far into the interior as Chiapas, which neighbors Guatemala.

Stating upfront that “the hard-up hinterland is all I have come to care about,” he steers away from beaches, pyramids and the “cyclorama of colonial cuteness” of San Miguel de Allende and focuses instead on regions ravaged by poverty and narco-warfare. The first 100 pages are episodic and gory, with Theroux rattling off cartel crimes as he goes “bumping along Reynosa’s side streets and potholed roads.” Once he reaches the central state of San Luis Potosi, though, he starts developing the passing landscapes down to the “skinny child chasing a skinny dog chasing a skinny chicken,” and by southern Oaxaca, he is capturing the essence of a person in a single sentence: “I sensed a melancholy in Magdalena, the slow solemn way she handled the baskets, her head tipped to the side, her sad eyes, a grimness at her mouth, the way she sighed when she got up to look for another basket, a heaviness in demeanor.” His observations about the autonomous regions governed by Mayan Zapatistas who have “resisted all that was negative and destructive” to build a movement “upon everything that was humane and enduring” are especially vivid.

Minus a few policemen, nearly everyone Theroux meets is friendly and eager to share whatever he needs: directions, dinner, a bed for the night and above all their story, often over mezcal. As he notes, “Mexican hospitality to gringos is in ironic contrast to the present ubiquity of Mexicans who are demonized and fenced in, stamped as undesirable, considered suspect, and unwelcome in America.” No matter that the Oval Office holder has derided them as criminals and rapists: “Mexicans spend very little time railing against the U.S. government, because in their experience, government by its very nature is corrupt, often criminal, and the poor are its victims.” When pressed about President Trump, they mostly shrug, “regarding it as beneath them to comment.”

Such revelations give this work its value, along with the fact that — as the best-selling author of 51 books — Theroux may have the star power to persuade someone who might not otherwise read a book about Mexico to do so and (the big hope) to care about its people.

Readers who already love Mexico, however, may have trouble making it past the fourth page. That is where, upon seeing a withered yucca on the side of the road, Theroux writes: “Like the despised Mexican, the person always reminded he or she is not welcome, whom no one ever misses: I could not be more sympathetic. I am this yucca with crazy hair and a bent back; I am also . . . a shifty migrant. Yo soy tú, I think. I am you.”

That’s right. An author deemed worthy of a gold medal by the Royal Geographical Society feels “shunned, snubbed, overlooked, taken for granted, belittled, mocked, faintly laughable, stereotypical, no longer interesting, parasitical, invisible to the young” and therefore “much like the Mexican” because — wait for it! — he had recently turned 76. We speak in literature about books having a “central conceit,” but this is ridiculous.

And the narcissism doesn’t stop there. Early in the book, Theroux opines that no other commentator, foreign or Mexican, has ever managed to “sum up Mexico.” While acknowledging this to be “a futile and dated enterprise,” he clearly aspires to do so himself. First, he lays out his credentials, dropping the titles of his previous works whenever possible and quoting from them at length. Then, as if taking a cue from the cartels, he guns down his competitors, calling a heralded novel by Yuri Herrera “portentous and incoherent,” accusing Laura Esquivel of “winking at the reader” as she wrote her classic, “Like Water for Chocolate,” and panning as “slight” Juan Rulfo’s “Pedro Páramo,” a novel ranked among Mexico’s finest. He even lambastes the genre that Nobel laureate Gabriel García Márquez made famous — magical realism — as “at worst a third-world writer’s affectation and at best a third-world writer’s indirection, like a magician’s trick of distraction.”

Equally unnerving is how Theroux attributes his own freedoms to happenstance. “More than fifty years of this, ever the fortunate traveler,” he sighs on the penultimate page. Theroux isn’t a beneficiary of luck but of privilege. There is a difference.

And yet, it is poignant when Theroux chides the master artist Francisco Toledo for calling himself old. They are the same age, and Toledo just died in September. Theroux seems worried this is his swan story, and he extracts such life-affirming joy from the road that you hope it keeps unfurling before him and, even more, that the wonderful people he writes about would be so graciously received during their own journeys to El Norte.

On the Plain of Snakes

A Mexican Journey

By Paul Theroux

Houghton Mifflin Harcourt. 436 pp. $30