Palimpsest is a piece of paper (or papyrus or parchment or the like) partially erased to make way for new text. It comes from the Greek for "scraped again." It's also not the title of a memoir by Lillian Hellman.
Pentimento is the underlying image on a canvas over which something new has been painted. It is the title of a memoir by Lillian Hellman.
They're great metaphors, especially when you are six years into a project of some large scale. You are working on a different book all the time, but a book that works well shows the progression. You pose questions that are answered later, surprising you (because if they aren't a surprise, you aren't writing but you're setting up straw men). As you edit, you realize that stuff you thought would be important isn't, and vice versa. Then there are also glimpses of an ur-book below it, the book you didn't write, and that's always sad.
James Joyce wakes up the night after he has turned Ulysses over to Sylvia Beach and he cries, "Goddamnit, I forgot to put in a yacht race!"
On the other hand, as you rewrite, you start to feel glimpses of something else peeking through. You start to address the obsessions you didn't have when you started. And these are either part of the book, or they're your next book beginning to be overlaid, badly, on the current one. So the palimpsest works both ways, trying to make sure the old book is integrated with the new one, and that the next book hasn't colored it too much.