After months and months of writing you finish the first draft, which is more or less where you actually leave the characters behind as living creations, but you still have to do rewrites so you can't celebrate yet. You do your second, third, whatever number of drafts, and when you get to the last one, well, in a sense that seems like it's really it, because the book is all but finished. You've worked and polished it. If it was a statue for the town square, you could put it up on its pedestal and let the pigeons start doing their jobs. But you can't really celebrate, because you still have to do the proofs.
Some effects disappear.
Page breaks become awkward in a way you didn't notice in your manuscript. Or just more material related to the book, like this thing you're reading now, which I'm writing two full weeks after the proofs have gone back to my publishers, but a couple of months before the book is actually published. I'm not quite sure. In fact, until other human beings are actually reading the book and reacting to it, I probably will still feel there's some part of the process unfinished. I'm completely sick of it.