Of course it is based on one of Jesus' most well-known parables, the Prodigal Son, found in Luke 15. As Keller says, in this parable "Jesus reveals God's prodigal grace toward both the irreligious and the moralistic. This book will challenge both the devout and skeptics to see Christianity in a whole new way." And I agree. I was pleasantly surprised to find so much new meaning in such a familiar story.
Right from the start he gives new definition (for me) to what it means to be "a prodigal." He says it is (1) one who is recklessly extravagant; and (2) one who has spent everything. We generally think of the "prodigal son" as the one son who went away and spent his money on wild living, and we believe this is who the story is meant to be about. But Keller does a good job of showing that the parable was actually directed more towards those "older brothers" among us (the Pharisees), and that it was actually God who is recklessly extravagant... with grace, and who spent all he had to... bring us home. He says Jesus' purpose with this parable is not to "warm our hearts," but to "shatter our categories."
Anyway, there are many great quotes and stories I took from this book. Rather than trying to pick a few to add here, just let me recommend that you read the book. It is short, it is easy to read, and it is very worthwhile. And, as Keller points out in the introduction:
Many lifelong Christian believers feel they understand the basics of the Christian faith quite well and don't think they need a primer. Nevertheless, one of the signs that you may not grasp the unique, radical nature of the gospel is that you are certain that you do.
I was probably in that camp - just as the older brother was - and I am glad I read The Prodigal God. It was good for me, and I think it would be good for more people - whether they call themselves "christian" or not.