Read this book in order to understand the culture of the South during and shortly after the Civil War--Faulkner's view of it, anyway. It's a collection of related short stories in chronological order, and the last story, "An Odor of Verbena," is key. "An Odor of Verbena," which concerns dueling, the demand for violent revenge to satisfy family honor, and the human cost of such a culture, was required reading in the Southern American culture course I took.
Flags in the Dust, Faulkner's other book (a novel proper) about the Sartoris family, was written first but is set 40-50 years later; Bayard Sartoris, the young protagonist of The Unvanquished, is an old man in Flags in the Dust. So read The Unvanquished first for an introduction to the Sartoris family. (I mistakenly did the opposite.)