Not so much a book length work as an extended sermon or meditation on the parable of the Prodigal Son, Rev. Keller uses the parable to get deep inside the operational definition of Christianity and to diagnose what is wrong with how the faith is practiced. He takes the interesting track of examining the parable with the elder son as the focus. While not denying traditional explications of the parable, Rev. Keller fleshes out an alternative understanding that shows how Jesus was speaking of two ways of "missing the mark." More significantly, Jesus shows us that one son is restored, but the elder son chooses to remain apart at the end of the parable--driven away from God by expectations of controlling Him.
Rev. Keller points out that among those who are faithful there are many reasons for the faith, not the least of which is, "What can I get out of it?" That is, many are faithful because of the promise of the inheritance, not because they truly love and worship God. These are the modern representatives of the elder Son, driven to distraction by the thought that God will redeem whomever He chooses and invite back into the fold those who have led dissolute lives. These are the Christians who want to draw lines between "us and them." Those who have faithfully followed the path their entire lives, and those "line-jumpers" who nevertheless manage to engage God's compassion and saving love. These "righteously angry" are not angry for the sake of righteousness, but angry because they live not in faith but in fear and expectation.
The explication of the parable explains a great deal that one can see in the Church and in Churches throughout the world today. The anger, the self-righteousness, the bitterness, and the soullessness. If we cannot join the party that welcomes the prodigal back, then we live in constant misery--desiring to control God and command God to meet our expectations. Faithful, not out of love, but out of fear (God will get me if I am not) or out of expectation (if I'm faithful, I'll get something good at the end and maybe before.)
I do a poor job of presenting the thesis. Encounter Rev. Keller's words yourself and decide. I found the book fascinating and convincing and perhaps even a little convicting. Highly recommended.