As I've mentioned before, I can't recommend the writings of John Piper highly enough.
The folks at Crossway books have done everyone a major service by offering absolutely free, full, online editions of his new book A God Entranced Vision of All Things: The Legacy of Jonathan Edwards, and his not-yet-released: When I Don't Desire God: How to Fight for Joy.
Piper has also made available full, free, online editions of his books A Hunger for God, Counted Righteous In Christ, Don't Waste Your Life, Desiring God: Meditations of a Christian Hedonist, God's Passion for His Glory, The Passion of Jesus Christ, and Recovering Biblical Manhood and Womanhood at his own website.
It's deep, challenging, soul-satisfying food, and it's all free. This stuff is a goldmine. I envy the person digging into it for the first time. You should download all of them to your computer and start reading them now. I'd start with either Desiring God, or the first four chapters of God's Passion for His Glory, which lays the whole foundation.