All the books available online at Desiring God's store will be on sale for $5 on Wednesday, June 27 & Thursday, June 28. If you've never read anything by John Piper before, check out the "Essential Piper" section. I've read and learned from Desiring God, Let the Nations Be Glad!, Don't Waste Your Life, and God Is the Gospel. I've read part of When I Don't Desire God, and have The Pleasures of God waiting in the wings. If you're up for some good meat, I'd recommend Desiring God. If you want something a little lighter in order to get a taste of Piper, I'd recommend Don't Waste Your Life. Or browse through all the books and see what's available.
While you're there, check out the resource library - there's a lot of good stuff there for free.
(HT: Desiring God Blog)