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.
At times all I need is a brief glimpse, an opening in the midst of an incongruous landscape, a glint of lights in the fog, the dialogue of two passersby meeting in the crowd, and I think that, setting out from there, I will put together, piece by piece, the perfect city… If I tell you that the city toward which my journey tends is discontinuous in space and time, now scattered, now more condensed, you must not believe the search for it can stop.
—Italo Calvino, Invisible Cities
What's up with the epigraph?
We could make this really short and sum up the epigraph with a cliché: it's not about the destination; it's about the journey. But like the Transformers, there's more to this than meets the eye.
You could look at the epigraph two different ways. One way is to see it as a description of Fawcett's journey for Z. Fawcett only has an inkling that Z exists, the "glint of lights in the fog," if you will. From his research on the jungle, he crafts a vision of a huge jungle city. Even without any concrete proof of Z's existence, Fawcett never gives up his search.
The other way of looking at the epigraph is from Grann's perspective. In a way, Grann is undoing all of Fawcett's work. Grann's journey all but proves that Z is "discontinuous in space and time." It likely collapsed and disintegrated long ago. There's a small but unlikely chance that the cultists following Fawcett are right and Z exists in a parallel dimension, but that's as likely as Megan Fox winning an Oscar for Transformers.