When doctrine starts to give you the impressions of armor you'll see what doctrine is. Then the old books will look like different parts of plate armor and weapons. And you'll be able to discern the true armor from the ill-made or just fake armor. The old books and the doctrine in them also carry impressions of the mysteries of the ages. And Biblical mysteries are truth. Often the books don't venture as deep as we'd like them to, but the basics of doctrine are strong and complete and can be meditated upon for deeper impressions.