The Kublai Khan sits in his garden while Marco Polo regales him with descriptions of the strange and wonderful cities he has visited. There is much unfeasible architecture, some odd anachronisms, such as an airport in one city, and some seriously post-modern touches, including our heroes discussing whether or not they are real.
The book bears a passing resemblance to some of Borges' short stories and it's similar in some ways to the other Calvino I've read, If On A Winter's Night A Traveller... Like If On A Winter's Night... each of the chapter titles can be combined to form something else, in this case a city skyline.
Each city is described in a short prose poem, none longer the four or five pages. Calvino groups the cities into themes - Cities and Memory, for example - but his descriptions are often only loosely connected to the theme at hand. In a wonderful display of imagination and descriptive prose, Calvino raises questions about how cities grow and evolve, how people live together and the limits of language.
It's a good book to dip into and one that will spark thoughts about all kinds of things but Calvino isn't really able to draw it into a whole. Instead he just stops. It would be frustrating in a longer work but Invisible Cities is short enough that he gets away with it. When it finishes, you'll feel like you've awoken from a series of very odd dreams.