A growing area of important study in Evangelicalism is that of spirituality. We have fallen way behind others groups in developing a thorough doctrine of spirituality. Over at Reformation Theology they have laid out the top 10 books on piety, sanctification, and spiritual growth. I agree with each one they have listed and encourage you to read each one carefully!
You can find their list here. These works will give comfort to the soul, inflame a life of holiness, and persuade a more intimate relationship with God.