The book explores imagination and the imaginable through the descriptions of cities by the narrator, Marco Polo. The book is framed as a conversation between the aging and busy emperor Kublai Khan, who constantly has merchants coming to describe the state of his empire, and Polo. The majority of the book consists of Polo's descriptions (1-3 pages each) of 55 cities. Short dialogues between the two characters are interspersed every five to ten cities and are used to discuss various ideas presented by the cities on a wide range of topics including linguistics and human nature. Not only is the book structured around an interlocking pattern of numbered sections, but the length of each section's title graphically outlines a continuously oscillating sine wave, or perhaps a city skyline. The interludes between Khan and Polo are no less poetically constructed than the cities, but form a framing device, a story with a story, that plays with the natural complexity of language and stories.
The book is probably based on The Travels of Marco Polo, his travelogue of the Mongol Empire written in the 13th century, which shares with Invisible Cities the brief, often fantastic accounts of the cities he visits, accompanied by descriptions of the city's inhabitants, notable imports and exports, and whatever interesting tales Polo had heard about the region.
One of Calvino's masterpieces, the novel does not fall under the aegis of magical realism, science fiction, or speculative fiction, and in fact is closer to poetry than classic novel writing. In the end, the book creates its own universe, neither that of a futuristic world nor one based on classic fantasy fiction (pagan myths, Christian folklore, etc.) nor does it obey E.M. Forster's classic model for the story, but creates a new form.
The book, because of its approach to the imaginative potentialities of cities, has been used by architects and artists to visualize how cities can be , their secret folds, where the human imagination is not necessarily limited by the laws of physics or the limitations of modern urban theory. It offers an alternative approach to thinking about cities, how they are formed and how they function.