Follow Don Winslow on Twitter, and you come to expect three things.

Advice and encouragement for aspiring writers. Reminders about his live events and publication dates. Links to horrific, soul-crushing reports on the never-ending 'war on drugs'.

It's no wonder that Winslow can't turn his eyes away from a tragedy that has claimed the lives of well over 120,000 in Mexico alone. He has spent the greater part of this century immersed in research about the rise of drug-trafficking syndicates south of the US border who feed addictions in the north - research that brought documentary weight to the pared-back genre prose of 2005's The Power Of The Dog and now does so again in that book's direct sequel, The Cartel.

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Winslow achieved other things in that period too. He more or less singlehandedly invented the surf noir novel with his detective tales set in and around San Diego, California and co-scripted the screenplay of his novel Savages for Oliver Stone's film of the same name. But it's the bloody history of the Mexican drug cartels that has become his obsession.

For that we must be grateful. Taken together as a 1200-page behemoth, The Power Of The Dog and The Cartel offer a riveting exposé of a modern tragedy where the fast pace of the thriller narrative never stumbles over the painstaking attention paid to detail and background. More importantly perhaps, they offer an alternative perspective on the accepted history of America's involvement in the 'war on drugs', a shocking litany of greed, complicity and right-wing political machination.

Ranging from the 1970s to the late 1990s, The Power Of The Dog referenced the Reagan-era drugs-for-guns deal with the Contras and Bush-era operations in Central and South America to massacre 'Communist' groups. Winslow placed both firmly at the heart of the US Government's acceptance, even behind-closed-doors support, of the rise of politically powerful Mexican drug syndicates. As he saw it, the 'war on drugs' had become the shameful public face for the funding of anti-Marxist conspiracies and the safeguarding of US business interests. Like the James Ellroy series that began with American Tabloid, Winslow uses what some would dismiss as mere pulp fiction to say important and damning things about the country he calls his own.

As did its predecessor, The Cartel opens with an attention-grabbing sequence from a point chronologically near the end of the book, then flashes back to reveal the network of circumstances leading up to that event. This time it's a US undercover helicopter attack designed to assassinate drug lords on foreign soil. That in itself suggests the stakes - political and personal - have increased since the closing pages of The Power Of The Dog.

The Cartel paints a picture of Mexico as a country where every aspect of daily life has been tainted by the drug industry; the desolation of Ciudad Juarez in particular, in just a matter of years, is heartbreaking. But within that big picture the story's narrative backbone remains the bitter, destructively co-dependent relationship between DEA agent Art Keller and his drug-trafficking nemesis Adan Barrera. Keller is utterly defined by his revenge obsession; he's a loner who moves among crowds, a symbol of the law who, as the book progresses, is ruthlessly willing to act illegally for his own ends. Barrera, equally ruthless, is the family man, gregarious and able to connect with colleagues from street level to government office even as he is separated from society either in prison or in his security-compound mansions.

There is no law of diminishing returns with this sequel. The Cartel is a much more graphically violent book than its predecessor, and that is absolutely the point: the most appalling body count in The Power Of The Dog is now a single afternoon's statistics in The Cartel. As civil war has raged on Mexican soil between rival drug gangs competing for the lucrative stretches of land that border on the US - drawing in corrupt local police and army officials, catching innocents in the crossfire - the sheer vindictive evil of those involved has toppled over the edge of what should be acceptable in even the most demented society. Victims are burned alive, skinned, decapitated. Winslow's descriptions are shocking, and so they should be.

We witness the effect on Mexico and, one step removed from the visceral, we witness the effect on Keller: a business like this can only turn the best intentions into the bleakest, blackest moral vacuum. As the DEA man consciously loses his humanity, Winslow's portrait of Keller's moral dilemmas (but also the chilling amorality of loose-cannon 'narcos'), contains his most powerful writing to date.

Ultimately, however, The Cartel isn't about regular heroes such as cops or government agents. Winslow reserves his greatest admiration for the journalists, the doctors and the women who stand up, at great risk to their own lives and those of their families, against the drug cartels' vicious rule. Winslow lays out their stories with the authority of an investigative reporter and the narrative skill of a best-selling author.

It's at the halfway point, when these civilian characters enter proceedings, that the book really starts to fly, as there is now much more emotionally at stake for the reader. And yet the author doesn't skimp on heart-stopping action set-pieces - an old man's last stand against the narcos at the ranch he has worked his entire life, a massive shootout in the Guatemalan jungle with a cast of hundreds. In the movies they'd say "the money is on the screen"; with Winslow, it's on the page.

As a reader, it's natural to want to immerse yourself again and again in the world Winslow depicts. But this, the author reminds us, is not wholly fiction: The Cartel is dedicated to a long but necessarily partial list of journalists murdered or "disappeared" during the period covered in the book. What's good for the reader, and hopefully good for Winslow's growing reputation as a thriller writer of remarkable moral depth, continues to be bad for the people of Mexico.