 What struggle does the door in the library represent?
There had been a revolt—again a term without any real meaning, a term that could have no reality outside of the reading machine—and the elders were overthrown. The people had decided that they did not know where they were going, or why, and that it was unfair that the elders alone had this authority. Now if the world had walked from—somewhere, through the vast star-speckled darkness and to the great garden outside, this also was purposeful. He had long wondered about the door in the back of the library, and now, as Crifer sat cross-legged on one of the dusty tables, reading machine and book on astronomy or stars in his lap, Rikud approached the door.
 The struggle for Rikud and all the others to conceptualize what they don't know or haven't seen before.
 The struggle between man and machine, and the power machine now has over them.  The struggle for Rikud and all the others to conceptualize what they don't know or haven't seen before.  Rikud's fear of what's behind it.  The struggle between authority and the people it runs.