They aren't lamps.
They hang overhead like lamps, and indeed they glow like lamps, illuminating the desks where contented readers pass the hours. No-one could be faulted for imagining that to be their sole purpose.
But they are collectors: of hopes and dreams, of highs and lows, of laughter and tears and anger and longing, of all the powers that a good book can unleash. The readers fill the air with it, and the collectors catch it, store it, keep it safe in crystal vials.
And when someone comes to the library depressed, or lost, or mournful, they sense it, and select a vial, and with exquisite care release a little light into a shadowed life.
So perhaps, in a way, they are lamps after all.