In 1972 the book "The invisible cities" was published.
The main characters are Marco Polo and 55 different Cities.
55 cities are described divided into 11 categories in each category there are 5 cities the cities are inserted in 9 chapters, they are inserted not grouped by categories. The first and ninth chapters consist of 10 cities, while from the second to the eighth there are 5 cities.
The categories are:
The cities and the memory, the cities and the desire, the cities and the signs, the subtle cities, the cities and the exchanges, the cities and the eyes, the cities and the name, the cities and the dead, the cities and the sky, the continuous cities, the hidden cities.