WHEN A CERTAING enoese mariner sailed southward to find the elusive Indies, he was guided by two favorite books. We know one as The Travels of Marco Polo: the product of a collaboration between that Venetian merchant-adventurer and his cellmate in a Genoese prison, a purveyor of popular romances named Rustichello of Pisa. This book was composed around 1298, in a literary creole designated today as “Franco- Italian,” but it was soon circulating widely in many different languages and editions. (There is no “original” text; the copy owned and annotated by that famous mariner was a later Latin translation.) The other book, attributed to one Johan (or John) de Mandeville, was published around the middle of the fourteenth century, in the French dialect then prevalent in England. It, too, survives in numerous variants, none of which is the “original” text. In its own time, the former work was often titled Le devisement du monde (The Description of the World), Le livre des merveilles (The Book of Marvels), or Il milione (The Million). The latter work was also known as Le livre des merveilles.

Along with their titles, these influential texts share some other salient characteristics: authorial and linguistic indeterminacy, a tangled history of transmission and reception, the tendency to imprint ancient imaginaries on a mesh of contemporary fantasy and observation. What were these books supposed to be about? How were they understood by generations of readers? To what extent do they constitute evidence for contemporary worldviews? Such questions are open to debate. In essence, these books are not travel narratives or practical manuals: they are mises-en-abyme. The mysterious conditions of their making underscore the challenges of knowing anything about the world they purport to describe—the difficulty, even, of talking about that world in terms that mirror its own conceptual categories. (“Franco-Italian,” “the French dialect of England,” the literary work as a fixed entity: all of these are anachronisms.) Yet like so many readers before me, I have drawn inspiration from one of these books.