Like music books are often connected to a memory, or a sensation, or an emotion one feels upon finishing; terrible regret, yearning, warmth, sadness. The best of books induce emotion, yet so do the worst. When do such emotions indicate a good book? When that emotion was identifiably the author's intent, or of a purpose and cause relevant to the content.
Some of the most powerful books induce in me a yearning. It is a yearning for the characters to become relevant, to somehow leap from the pages into reality and engage in a series of illuminating conversations with each other. It is a yearning for the fulfillment of the author's universe, for it to leave some print, something more that merely trivial, on the world. The dreary, less-than-perfect real world more complex than even the most cynical of tomes can effectively describe.
To experience a yearning after finishing a book is to subscribe to the ideal that a book represents. To someone reading a historical novel it is that desire to relive moments enshrined in the past that we can never visit save through the weavings of another's words; to read Harry Potter is to yearn for Hogwarts and magic, escapism into a world inherently magical and exciting, despite the conflict that is so central to the plot progression.
I have always associated the beautiful classic Watership Down with sunlight and gentle wind, quietude on a summer day, despite the fact that I don't know what summer is like, save as an observer in the goings on of the rabbits in the book. And I yearn for the characters to be real, that what has taken place, the heroism and the romance and the tragedy, even, has somehow taken place, has somehow assumed the label of reality, of being real, not merely in the mind, but in the flesh. The stuff of human thought sprung forth into the world, sharing a place with the real and the dull, infusing some life into tired senses.
Perhaps that is why fiction is such a terrible thing; good fiction and its contents are ever fated, like an unrequited lover, to suffer entrapment in the medium of mere words, when the words themselves are like cages of a real beauty, that, like a marble sculpture or an engraving, show off the fine curves of their charges in all their splendour while all the time keeping them from bursting forth into life. And it is up to the reader to imagine that it happens.
And yet, had not the craftsman made the statues, the reader would never even have the pleasure of imagination.