Killers of the Flower Moon by David Grann

In the 1920s the richest people per capita in the world were members of the Osage Indian nation in Oklahoma. After oil was discovered beneath their land, they rode in chauffeured automobiles, built mansions, and sent their children to study in Europe.

Then, one by one, the Osage began to be killed off. The family of an Osage woman, Mollie Burkhart, became a prime target. Her relatives were shot and poisoned. And it was just the beginning, as more and more members of the tribe began to die under mysterious circumstances.