The book works better if I know everything I can about the ending. Not just what happens, but how it happens and what the language is; not just the last sentence, but enough of the sentences surrounding that last sentence to know what the tone of voice is. I imagined it as something almost musical. Then you are writing toward something; you know the sound of your voice at the end of the story. That's how you want to sound in those final sentences: the degree that it is uplifting or not, the degree that it is melancholic or not.