Moss Hart once said something like, "You never really learn how to write a play; you only learn how to write this play." Jacques Barzun once disputed the idea that in art there are problems to be solved, since that implies a singular solution for the same problem everywhere; rather, he asserted, there are difficulties that must be overcome, and not just once but constantly. Much of what Barzun has said about art I've come to disagree with —The Use And Abuse Of Art is a frustrating mix of one truly good insight to about four or five old-man-yells-at-cloud-isms — but this point I rather like.
Every single time I start a new story, I'm starting from scratch in more ways than one. I've never written a sequel to anything, and that's pretty consciously the case. Every time I continue an old story, in some way, I'm missing out on all the other things I could be exploring new. So I throw it all out, because each story demands that I approach it with as blank a slate as I can manage.
But I can't really throw it all out. There's the accumulated experience of all the previous books: what to do, what not to do, why to do or not do any of it.
The old line in Zen is that you have to empty your cup before you can fill it again — that you have to discard all your assumptions about things and face them as unassumingly as possible before you can really understand either them or yourself. Even Yoda knew this: Unlearn!
What carries over, then? If nothing else, the process — the way we pay attention to details, the way we filter the bad from the good from the great, the way we gain an intuitive sense of how to advance the story or bring forth character. The process matters more to the creator than the artifact does; the artifact is just the road marker for their progress.
I once dreamed I had undertaken a task as a writer that seemed all but impossible: to start from absolute scratch. I remember standing at a window, looking down at a cityscape, and saying to myself, "Let's get rid of all of it. Character, story, plotlines, symbolism, metaphor, allegory, all of it. Let's wipe it all out and see what comes from the vacuum, because maybe that's the secret of nothing: it can't really exist." But then the dream dissolved and became something else, as dreams do, and on awakening I realized you can't begin with absolute zero. You always carry some nonzero with you; you embody it. "Whatever we do," John Cage complained to David Tudor, "it always ends by becoming melodic." Maybe that wasn't the worst thing.