The Prodigal God is by far one of the best books I have ever read. If you want a fresh but biblical outline of sin, man, and the Gospel, then this book has what you need.
Mark Dever: "This brief exposition is unsettling and surprisingly satisfying. Like seeing something as familiar as your own home, or your own self, with new eyes."
John Piper: "The Prodigal God is vintage Keller - biblically faithful with a fresh angularity. That is, Keller seizes a great truth and comes at it from an angle and with language that help us see it in new ways."
Keller has now put together a 6 week sermon series designed for churches and small groups to go through along with the reading of the book. The official site has the sermon audio for free, but you can also check out the store to pick up the leader guide, DVD's of the sermons, and the books. I highly recommend this book and plan on using this for my own small group.