As the actual human reader of If on a winter's night a traveler, you'll probably be able to identify pretty closely with this one. Over and over, Calvino offers you really interesting beginnings to novels, only to break them off just when they're starting to get good. And it's all crafted, too—it's a technique he's using to show how pleasure is connected to your sense of potential in what you're reading.
As much as you, the Reader, are disappointed with books in this text, your way with the ladies is no different. You're continually attempting to find concrete points of connection with Ludmilla, only to see them swept away by her pesky insistence on being her own person (what nerve). This sort of disappointment arises whenever you try to make the world mean something, then have it constantly contradict your reading. Ugh, world.
The "disappointment" experienced by the Reader isn't actually disappointment at all, but rather a prolonged feeling of excitement.
Calvino's text suggests that Readers who come to books expecting a clear, straightforward plot are ignorant and don't deserve to feel satisfied.