...then realized the book wasn't ready to be queried, stopped querying, only then you had a new book, totally ready to be queried, that's perfect for a different agent at an agency you queried the first book at. Suppose also that you never heard back from the other agent about the first project but you're sure it's a no because it's just a bad book. Could you email the first agent, say "I'm withdrawing this", then query the other agent at that agency about the new book? Or do you just wait and wait and wait until the old query has gone stale and been forgotten, then query the new book with the new agent?