NEW YORK (AP) — Isabel Wilkerson's first book since her Pulitzer Prize winning “The Warmth of Other Suns” is a years-long project that will explore what she calls the “unseen skeleton” of hierarchy in American life.

Random House announced Monday that Wilkerson's “Caste: The Origins of Our Discontents” comes out Aug. 20. In the book, Wilkerson writes that “The human impulse to create hierarchies runs across societies and cultures” and “predates the idea of race.”

“Caste is the bones, race is the skin," writes Wilkerson, in an excerpt provided by her publisher. "Race is what we can see, the physical traits that have been given arbitrary meaning and become shorthand for who a person is. Caste is the powerful infrastructure that holds each group in its place.”

Wilkerson's “The Warmth of Other Suns,” her acclaimed work on African American migration from the South in the 20th century, was published in 2010. The debut book won the Pulitzer and National Book Critics Circle Award, its admirers also including President Barack Obama, who presented Wilkerson with a National Humanities Medal in 2016 for "championing the stories of an unsung history."