I actually *gasp* bought this book from Amazon for full price. It seemed to have good reviews and a good theme; I had it recommended from the last book I finished, and I've identified with the story quite a bit over this last year. I was a little dismayed at the length (especially relative to the price), since it was such a short book. At least it was clear and to the point.
The main gist is that the story of the Prodigal Son is actually a Story of Two Sons. Both sons actually fail to obey / respect their father who reaches out to seek reconciliation with both of them. The older thought that by keeping all the rules they'd be saved, much like many judgmental or overly legalistic Christians.
Also, the term 'prodigal' doesn't mean wayward, it means "spendthrift" or "someone who gives something away lavishly". Hence... a Prodigal God who gave away grace completely freely.