[SOLVED] How to create a queue of spawning game objects after one another and stopping based on a countdown timer

Questions
5 
local function create_book(self)
	-- attempt to call global 'delete_book' (a nil value)
	delete_book(self)
	self.book_id = factory.create("#random-book-factory",  vmath.vector3(-1.0, 0, 0))
	
	go.set(self.book_id, "position.x", 0.0)
	go.animate(self.book_id, "position.x", go.PLAYBACK_ONCE_FORWARD, 100, go.EASING_INQUART, 2)
	sound.play(self.book_id.."#sliding-sound")
end

local function delete_book(self)
	if self.book_id then
		go.set(self.book_id, "position.x", -1.0)
		go.animate(self.book_id, "position.x", go.PLAYBACK_ONCE_FORWARD, 100, go.EASING_INQUART, 2)
		sound.play(self.book_id.."#sliding-sound")
		go.delete(self.book_id)
		self.book_id = nil
	end
end

So I’m getting an error -- attempt to call global 'delete_book' (a nil value), but I don’t think that should be happening since there’s the if statement, so I’m confused?