John's first book, Entering Hades: The Double Life of a Serial Killer (Sarah Crichton Books, FSG), was named an Editors' Choice by the New York Times Sunday Book Review and a Men's Vogue Best Book of 2007. It went on to inspire The Infernal Comedy, the stage production starring John Malkovich.

His second book, Cold a Long Time: An Alpine Mystery, won the 2012 Independent Publisher Award. The German translation, Eiskalter Tod, published by Residenz Verlag, became a bestseller in Austria.

John's investigative work on the Jack Unterweger, Duncan MacPherson and Angelika Foeger cases has featured in television documentaries produced by A&E Biography, Discovery, the Canadian Broadcasting Corporation's Fifth Estate, and the Austrian Broadcasting Corporation's Am Schauplatz Gericht.

His book The Vienna Woods Killer tells the story of Jack Unterweger, a celebrated Austrian writer once held up as a model of criminal rehabilitation, who turned out to be responsible for a string of murders across Europe and southern California. Drawing on privileged access to Unterweger's diaries and the cooperation of those who knew him, John pieced together what the FBI would later describe as history's only recorded example of a transatlantic serial killer.

John grew up in Highland Park, Texas, down the street from Sandra Bridewell, who fell under suspicion in 1986 for murdering her third husband and possibly two other victims, though she was never arrested. John began a multi-year investigation into the case in 2007, and his 2023 book, The Meaning of Malice, presents extensive evidence of Bridewell's guilt, making it the first true crime book to build a forensic case against an officially undetected serial killer still living and at large.

In 2024, John took a break from writing to produce and direct his first feature documentary, Double or Nothing, which follows a former bookie's high stakes wager to run the Honolulu Marathon despite being badly out of shape, and the friendship and possible cheating that unfold along the way.