The Look! Store your Books in the Rafters Post was a pretty popular one, so we wanted to update everyone with what's been done for lighting:
By adding these Grono Lights from Ikea ($6.99 each) in alternating cubby spaces, we've added a soft glow to the room. And since the lights use CFL bulbs, they don't heat up like a normal bulb would so we aren't as afraid of a fire hazard. Although we turn them off if we're not at home.