I should've said it's what I imagine children leaving home might feel like - don't have any myself! Is it a cliche to say my books are my babies? Oh dear.

Not a cliche at all! It must be very similar - they grow, get more developed..and then vanish from your life and you sort of worry about how they are faring, if they are doing well, if they have a good future ahead of them, if you did well enough bringing them up..as metaphors go it must be very accurate! I hope mine make their way in the world and grow up big and strong..