In The Prodigal God Tim Keller "lays out the essentials of...the gospel" by examining Jesus' parable commonly known as "The Parable of the Prodigal Son". As you can tell from the title of the book Keller has a different take on the parable than most other preachers.
Keller contends that the parable isn't merely about the "prodigal" son (younger brother). Rather it's a story about two lost sons and a father who's love extends to both of them. It's a story in which Jesus shows that the Gospel is for both the outwardly sinful and the religious because both are in desperate need of salvation.
I think that this book, like Keller's previous book, will appeal to both believers and unbelievers alike since it focuses on a message that both groups need to hear.