Right now, DesiringGod.org has all of John Piper’s books for $5.
Be patient, as you would expect, the server is running very slow.
Personally I would recommend:
The Pleasures of God
This is my favorite John Piper book ever. Nothing has helped shape my view of God’s glory, and self-sufficiency, than this great work on what God takes pleasure in. Nothing has ever encouraged me to read scripture to take great pleasure in God.
God is the Gospel
When it comes to reformed views on the gospel, most people find it inapplicable to daily life and/or not practical for actually missional/evangelical work. This is a excellent work that treats the gospel as foundational and exhaustive to the Christian life. I find it a great study to help live on mission in this world. There is a reason Acts29 places this book in their missiology category of recommended books.
Recovering Biblical Manhood and Womanhood
A great selection of essays for a Biblical foundation of the roles of men and women. Plus, I have a man crush on Wayne Grudem. There I said it.
(Ryan, your recommendations?)