The reader has certain rights. He bought your story. Think of this as an implicit contract. He’s entitled to be entertained, instructed, amused; maybe all three. If he quits in the middle, or puts the book down feeling his time has been wasted, you’re in violation. — Larry Niven, American science fiction writer
Even if the reader doesn’t ‘buy’ your story (perhaps the book was a gift, or borrowed from a library), a writer still must play fair. You can’t advertise your novel as the family-friendly tale of a dog trying to get back home, for instance, if it’s really a steamy romance from the dog owner’s point of view.