Piper has said that if you've read "Desiring God" there is almost no need to read his other books as they all are refrains of this work.
And that is the truth too.
It is true, somewhat.
Desiring God is like a prolegomena to Piper. It gives a basic understanding of his catch-phrase "God is most glorified in us when we are most satisfied in Him." It introduces God's pleasure in Himself, God's pleasure in His plan, and God's pleasure in us being pleased in Him.
Other Piper books, such as,
The Pleasures of God and
Future Grace go more in-depth theologically, while books such as
A Hunger for God (which deals with fasting),
Let the Nations Be Glad (which deals with missions and evangelism), and
When I Don't Desire God (which deals with ups and downs of progressive sanctification) are more practical in nature. He also has excellent biographies, a good thesis-style handling of Romans 9, and very good devotionals.