Keller's interpretation is also commendable for the balance maintained between the individual and cosmic levels of the parable. Yes, the story is about individual redemption for both the wayward libertine and the self-righteous Pharisee. But the parable also tells the larger story of the human race's exile from the garden of God. It is not merely a story of individual salvation, but of cosmic redemption through Jesus Christ for all of the sin tainted creation.
Second, as indicated above, Keller does a fine job of maintaining rigorous doctrinal orthodoxy. A few examples will do to demonstrate.
Consider the author's treatment of sin. Keller suggests that the popular concept of sin as rule breaking doesn't really do justice to the biblical doctrine. Now when I hear of an author who wants to posit an alternative view of sin, I usually prepare myself for a truncated view. However, Keller does nothing to lessen the gravity of transgression and the seriousness of sin's consequences, namely alienation from God. He suggests that sin is really a matter of the much deeper issue of self-righteousness. In this understanding of sin, one can keep every rule and still be deep in sin, because the central issue is trusting oneself for salvation rather than Christ.
Also worthy of commendation is Keller's treatment of the Christian hope. He steers clear of any neoplatonic pie in the sky ethereal escapism. With the Old and New Testament prophets, Keller outlines a vision of a very material and this worldly salvation. He is firmly committed to the divinely declared goodness of creation and points forward to the ultimate end of humanity's exile from the garden when the city of God fills the earth and access to the tree of life is restored. The goal is the very material wedding feast of the Lamb, as indicated in the feast hosted by the prodigal father in Jesus's parable.
I cannot give this book a higher recommendation. It is an outstanding introduction to the essentials of the Christian faith. Christians and non-Christians alike will benefit from this lucid text.