One day a few years ago, 300 migrants were kidnapped between the remote desert towns of Altar, Mexico, and Sasabe, Arizona. A local priest got 120 released, many with broken ankles and other marks of abuse, but the rest vanished. Oscar Martinez, a young writer from El Salvador, was in Altar soon after the abduction, and his account of the migrant disappearances is only one of the harrowing stories he garnered from two years spent traveling up and down the migrant trail from Central America and across the US border. More than a quarter of a million Central Americans make this increasingly...