The book is an examination of Jesus' parable from Luke 15:11-32, commonly called the parable of the Prodigal Son. Keller seeks to examine the parable to see what it teaches us about the nature of the Christian faith, and he exposes a parable that reads very differently than most people have thought. Rather than focusing on the wayward son, Keller points out that the focus of the story is actually the father, who represents God to us. Also, he spends a large amount of time examining the older brother in the story, who is actually cast in worse light than the younger brother. This was especially helpful to me, pointing out how dangerously often my "obedience" to God is really self-centeredness in clever disguise. In the end, though, the lasting impact of the book is a picture of a father who seeks to reconcile both sons to himself and does so at great personal cost. This is the inspiriation for the book's title - rather than meaning "wayward" as is often assumed, "prodigal" actually means "to spend recklessly, to spend all that one has." Keller paints us a picture of a God who is the ultimate prodigal, who takes the debt of sin on himself and who graciously and lovingly calls both individualistic, rebellious "younger brothers" and self-righteous, self-seeking "elder brothers" to repentance.
I really cannot recommend this book strongly enough. Keller writes extremely well and makes his points with clarity and power. Last night, I was talking with two of my grads and they were both gushing over the book. One of my guys was nearly finished with it after just 3 days, adding that he'd never finished a book in a week in his life. Whether you're a high school senior or a senior citizen, this is a book that you'll find edifying, enlightening, and encouraging. Put it at the top of your summer reading list.