First, look up the word "prodigal" in the dictionary. Then, you will see clearly how Tim Keller can call God the prodigal. This is a most insightful book into the mind and heart of God and how sinners need to respond to His generous salvation.
I loved the way Tim Keller showed me there were two victems in the prodical of the lost son. I learned that it's just as bad a sin to be obedient but with the wrong attitude. It shows the love of God in a more personal way. I plan on leading a Bible Study with this.
This book does a great job of explaining the essence of the gospel. It reveals the two most common ways that people tend to rebel against God. Most of us are familiar with the way of the younger brother, but the emphasis on the elder brother was new to me and very helpful. The elder brother is those of us who seek to get what we want from God by means of obedience. But we should want God himself, not what we can get from him.
It is a good discussion starter for small groups. The parables hold so much information that we can discover, and Timothy Keller has done a good job of disecting this one to give us a fresh look on an old story.