Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
All America lies at the end of the wilderness road, and our past is not a dead past, but still lives in us. Our forefathers had civilization inside themselves, the wild outside. We live in the civilization they created, but within us the wilderness still lingers. What they dreamed, we live, and what they lived, we dream.—T. K. Whipple, Study Out the Land
What's up with the epigraph?
The epigraph feels like the reason Larry McMurtry wrote the book, since it's not anything the characters in the book would know or quote. It's a book that looks to the past and tries to make sense of it—and to make it alive. As Whipple says, "our forefathers had civilization inside themselves." Gus and Call are our ancestors, and as Texas Rangers, they were fighting to establish a civilization, the civilization we live in today. But "the wilderness still lingers" within us, which is why books like Lonesome Dove, which are windows into the wild past, still appeal to us even today.