Ester (Chapters 1-5)
We can learn many things through this amazing story.
From the king we learn the importance of not making rash decisions.
From Mordecai, we learn that our actions affect more than just ourselves.  He had to live with the fact that his actions put his entire nation in jeopardy.  Fortunately, God had a plan and a purpose in the situation.
From Ester we learn the importance of being prepared before heading into battle.  What is the most important way to prepare spiritually for battle?  By prayer and fasting.  There is great power there and most Christians do not understand the significance of these weapons.