In a note appended to The Castle of Crossed Destinies, Italo Calvino says:
I would like to add that for a certain time it was my intention to write also a third part for this book. [There follows a description of this planned part.]
I went no further than the formulation of the idea as I have just described it. My theoretical and expressive interests had moved off in other directions. I always feel the need to alternate one type of writing with another, completely different, to begin writing again as if I had never written anything before.