DEA agent Art Keller, introduced in 2005's The Power of the Dog, resumes his personal campaign against illegal drugs in The Cartel.

Why have I returned to the subject of the "war on drugs"?

Other than my wife, son, and a few close friends, I have spent more time with one person than any other. Art Keller and I have shared 11 years of my life, and more than 40 of his.

The only thing is, he doesn't actually exist: Keller is a fictional character in two of my novels, The Power of the Dog and its sequel, The Cartel. He starts this saga as an idealistic young DEA agent assigned to Mexico. In the course of the first book, he loses his partner, his family, and his ideals while trying to bring down drug lord Adán Barrera. Keller finally succeeds, and, with Barrera in jail, he retires to a life of quiet contemplation.

When the next book starts, Barrera escapes and Keller must hunt him down. He just won't give up.

I know the feeling.

After I finished Dog, I never thought I'd go back to writing about the war on drugs. Then the events in Mexico spiraled out of control. The more I saw of the escalating violence, the all-permeating corruption, the ever-worsening drug epidemic in the United States, the more I knew that I would write the story.

I came back—with my old friend Art Keller.

Keller is a complicated man with a simple mission that he simply won't quit: to bring down a man he regards as the ultimate evil.

He'll do anything to achieve that end. And yes, to him the end does justify the means. As such, I think he's an American character—an Ahab plying the seas, an Ethan Edwards riding the prairie. I'm certainly not comparing The Cartel to Moby-Dick or The Searchers (or myself to Herman Melville or Alan LeMay—or John Ford, for that matter), but as Ahab was pursuing a white whale, Keller is after the white powder of cocaine and heroin, and the man who traffics it. Ahab lost a leg; part of Keller's soul has been sacrificed. Edwards searches for a kidnapped little girl; Keller searches for a lost innocence. Edwards is fueled by hatred; Keller by revenge.

I like Keller: he's complex and full of contradictions—a gentle man capable of violence, a moral (in fact, religious) man capable of treachery, a loner who, in The Cartel, falls deeply in love with a woman who is in all ways his match.

At the end of Dog, I left Keller pretty much where I left myself, seeking a more peaceful existence away from the war on drugs. Then events compelled me to write the next part of this story, and as I tried to figure out how to do that I realized that I had to bring Keller with me.(Or he brought me.)

I knew that the next part of the story had to be the furtherance of Keller's quest. I knew that I had left it unfinished, just as the drug war in Mexico was unfinished. Only Keller and his obsessive quest to finally bring down Adán Barrera could complete the epic.

There are other characters in the book—many of them. But Keller is the heart of it.

It was good to be back with my old friend.