Speaker for the Dead is set in the far, far future, when humanity has spread out across the universe. Most of the novel occurs on the planet of Lusitania, but you also get to spend a little time in the city of Reykjavik on Trondheim, an icy Scandinavian kind of world.
Lusitania, the setting for almost the entire novel, is a puzzle. The human colony is isolated in a small Catholic village surrounded by a fence, with life centered around the Church and the monastery; it feels almost like a medieval hamlet. Outside, the piggies, or pequeninos, whose habits and beliefs are almost entirely unknown, live in the forest. By the river, eels mysteriously appear, without any clear method of reproduction. And then there's the Descolada, a native virus which killed many of the colonists before Novinha's parents, the colony's xenobiologists, managed to find a cure. But the disease's exact nature is still uncertain.
The Lusitanians, then, don't know their neighbors; they don't understand their landscape; they don't even know their own bodies, in which the Descolada sits, neutralized but not understood. They may live way out on a distant planet, but they're still über-yokels, who don't comprehend their own innards, much less what's out there past the fence. Much of the setting of the book is essentially a big sign saying, "Here Be Monsters. Or Eels. Or Something."
The person who solves the puzzle and draws the map is Ender. Reading the book is sketching in Lusitania; Ender ends up speaking not just Marcao, but the whole planet. You could say Lusitania is the book before you read it: unopened, uncertain, filled with questions. And as Ender reads it, it becomes not a puzzle but an explanation. Reading let's you see the world, which in this case is Lusitania.