In If on a winter's night a traveler, you, the Reader, are constantly being pulled along by a conspiracy that works in ways you don't understand. And you have no say in the matter. Every time you act or think, you have to wonder if this is the way you've been intended to act or think by some higher power. This question finds its biggest expression when Silas Flannery wonders if his thoughts are secretly being dictated to him by aliens. (Um, what?) It might seem really weird and out of place, but it refers back to the larger question of how much free will you actually have as a human being—especially when you're immersed in the world of reading. In this world, interpretation might belong to you, but the world itself has been created by some author you've probably never met.
In If on a winter's night a traveler, Calvino suggests that you have just as much control over what happens in a book as you do in real life. It all depends on how you look at it.
By constantly talking about how wonderful the pleasure of innocent reading is, this book basically teaches its readers to take a more passive approach to life, and to accept whatever happens to them with a smile.