I'd like to suggest that instead, the site use local storage to remember what the last chapter downloaded was, and offer to only download updates.
I googled it and can't find how that works with the Kindle. Do you have a link I can read on the update process?
it just treats it as an entirely new book. What I'm suggesting, then, is that ficsave remember if the fic has been sent to the kindle before, and if it has been, then when the user sends it to the kindle again, offer to the user that it send a version of the fic that has nothing but the new chapters. that way the kindle will end up with two books, but one of them will be the first half, and the other will be the continuation.
Is that how it works? That's good to know. Would it be a lot of clutter to have multiple books with different chapters in them, rather than let's say, deleting the old book so you only have one copy?
It might be, but when you're downloading a lot of fics, it's easy to lose track of what you've read that way - kindle keeps a "where you were" marker, and if you download a new copy, it won't know that you were reading. So then when you go back to your kindle, you will have two copies - one a ways back in your history, since you probably read other stuff since then, and the new one that looks like it's never been read. What I really want is a way to update the old one, but I'm pretty sure kindle doesn't let you do that, so it seems a bit less confusing for new copies to not have the parts that are in the old ones.