John Piper's book Desiring God is available for an excellent price at the Scripture Truth Book Company for the low price of $10.39
I mention this book because it had a profound influence on me and my understanding of the sovereignty of God and what God requires of us.
From the Publisher:
Scripture reveals that the great business of life is to glorify God by enjoying Him forever. In this paradigm-shattering classic, newly revised and expanded, John Piper reveals that the debate between duty and delight doesn’t truly exist: Delight is our duty. Readers will embark on a dramatically different and joyful experience of their faith...The pursuit of pleasure is not optional. It is essential.