How do I know that a book is finished? A book has a hundred endings. There's an ending of sorts when everything's been said the first time round, but that's rarely a real ending, or even the beginning of an end. It's something to start from. And then there's lots of redrafting and re-groping towards some kind of a sense that the book is getting smoother, more complete. And there's a final ending when the time has simply gone and the book has to be finished because it can't go on and on forever being redrafted and rethought. There's a danger here, because the initial impetus goes, and the polishing replaces the passion.
Most of ending a book doesn't mean re-ending the last sentence or even the last chapter. It means rewriting those characters or paragraphs or dialogues where there are ugly spikes, or which don't work hard enough, or are dead or over-fancy. This too could go on and on forever. I do know, I think, that the best endings are intrinsic in the beginning, that the reader sighs with satisfaction, like switching off the lights at the end of a really wonderful day.
For the record, one of the best endings I know is Penelope Fitzgerald's 'The Beginning of Spring'.