As you can see on the right, I've just finished reading The Prodigal God. Because I've just been reading it, I've been thinking about it a lot, but let me just say that it's a great book. And thus, I wanted to use this forum to pass on the recommendation.
It's subtitled Recovering the Heart of the Christian Faith, and I think that's an apt description. Tim Keller uses the parable of the Prodigal Sons to drive home the true message of the Gospel. If you're looking for a great synopsis of what the Christian message is all about, I highly recommend this book.
And if you think it's possibly mis-titled, you definitely need to read it.