Tim Keller writes in a clear and engaging style. I particularly like the way that he illustrates his points by drawing on other writers. The overall effect is quite persuasive.
However, some of the clarity comes at the expense of over-simplification. I think most of us can identify with the older/brother categories, but people are far more complex than that. In this case, two sizes do not fit all. As it is, the first part of the book feels like a sermon on the dangers of dry, uptight religiosity. It is a good sermon, but I wonder how urgent a pastoral issue this is outside the US.
Keller goes on to broaden his message from this one parable without considering whether Jesus's original listeners would have heard it in that way. He does, rightly, locate the parable as a parable of exile and homecoming, but seems to struggle to make up his own mind as to precisely what the homecoming is. Did Jesus inaugurate the kingdom - albeit with a future consummation - or is it a still future event, as the final chapter implies. In which case, what exactly did Jesus achieve? On the one hand. Keller says that Jesus defeated the power of "death, disease and disorder" generally (nice alliteration), but on the other hand, this is BECAUSE (my capitals) Jesus died to pay the price for MY sin. Keller has smuggled in an atonement theology judicial role for God; a role that sits uncomfortably with the scandalously extravagant love of the father of the prodigal (Deuteronomy laid down a death penalty for stubborn and rebellious children). As a minor quibble, I was disappointed that he omitted the critical qualification that SOME (again, my stress) "Christian theologians have spoken" about Jesus's sacrifice securing the necessary not guilty verdict.
There is good practical advice within the book, but, in contrast to the essentially corporate nature of the kingdom, Keller's vision feels very individualistic. It's all about my relationship with God, my "inner-heart conviction", me sensing God's forgiveness, me "absorbing" the gospel. He skips over the role of the Spirit in both calling people to faith and equipping them for life within the body of Christ. For Keller, church (or at least the right kind of church) is primarily a useful support to us in our development rather than the purpose for which we are called and equipped, and the place in which the present inauguration of the kingdom should be experienced. The role of the sacraments is dismissed in a single sentence.
Lastly, an author ought to be careful about quoting another writer to support one point in isolation if this is inconsistent with the author's overall arguments. CS Lewis did speak of Sehnsucht, but held a quite different understanding of salvation from Keller. Bonhoeffer addressed worshipping communities (in established sacramental churches) in the face of the pressing evil of Nazism. He wasn't advising individuals on their spiritual development.
This is not a bad book. Maybe it will encourage some readers to come to faith for the first time because it is such an easy read. My feeling, however, is that, fundamentally, it only preaches to the converted. They seem to love it. I'm not one of them. There are better books on basic Christianity, including CS Lewis' "Mere Christianity". The style may now be less engaging, but there's more substance behind it.