Pastor-Scholar John Piper is a former professor of New Testament at Bethel Seminary, the Senior Pastor of Bethlehem Baptist Church in Minnesota, the head of Desiring God Ministries and an award winning Christian author. The web page for Desiring God was already one of the best web sites on the net featuring an extensive archive of audio resources and articles by Piper but now many of his award winning books are online as well. Among the free titles available for download are "Desiring God", "What Jesus Demands from the World" and "Don't Waste Your Life."
They are all well worth reading although some of them, like "The Future of Justification," is tough going if your not familiar with the current state of Biblical studies.
Check them all out here: http://www.desiringgod.org/ResourceLibrary/OnlineBooks/ByTitle/