Lackless riddle/candles Not sure if it's been discussed before, but another reason for keeping candles out of the library could be from keeping candles away from the four-plated door.

Sorry I'm not providing any evidence. If I recall correctly one of the lackless riddles or both mention a candle and somehow relating it to Haliax/Lanre. Assuming Haliax is some how behind the door, if Kvothe brings a candle-maybe a very specific candle-to the door it will help open it.

Just a random thought