John Piper is an American pastor who has written and published a number of books and articles that help to explain and illuminate biblical truth. Some of these works he has made available as free downloads from his website:

www.desiringgod.org/ResourceLibrary/OnlineBooks/ByTitle/

One of the books that I have recently downloaded is called 50 Reasons Why Jesus Came To Die, and I plan to read and discuss this with my small group soon. There is even a leader’s guide for group study!