Justin Taylor compiled an awesome list of recommended shorter works (sermons, articles, essays, etc.) named by prominent evangelical leaders as having special impact in their life and ministry. The list is a treasure trove of wonderful resources.
In the spirit of the inquiry, here's my own list:
C.S. Lewis, "Myth Became Fact," in "God in the Dock": Essays on Theology and Ethics
Augustus Toplady, "Thoughts on the Assurance of Faith," The Complete Works of Augustus Toplady
Tim Keller, "What is Gospel-Centered Ministry?," Sermon delivered at 2007 Gospel Coalition Conference
Ralph Erskine, "The Difference Between Legal and Gospel Mortification"