A rook is a black bird that looks like a crow. It's the kind of bird that likes to hang out in groups on bare tree branches and spook people. For the speaker, the rook is a means of connecting to Charles Lamb despite the distance that separates them. All the other rooks have gone home, and this one is the last straggler. It's that old ploy: if you can't join 'em, pretend that you have attached a lucky charm to a random bird that passes overhead and hope that the bird will sprinkle luck on your friend. Yes, an old ploy….