Just re-read my opening post and it sounds confusing (too much read/unread), so:
What I'd like to do is to mark everything on the reader as "unread" (thus emptying the predefined "Unread books" collection) and then to mark specific books I'm planning to read soon as "unread", so that that collection contains *just* those books and no others.
I know I can create my own "Books to read next" collection, and that's what I'm doing now. It'd just be nice if I could use the predefined collection instead, and incidentally strip off all of those "NEW!" icons from the book covers of everything I've just loaded onto the reader.