Earlier this year, I read two books on the so-called parable of the Prodigal Son. Actually, I agree with Keller that there are two lost sons, but that the Father is the central character... One of the things I love about the parable it is that it's possible to preach so many sermons on different aspects of it without too much stretching. Actually, I think that's probably possible on many bits of the Bible, just we don't bother with most of it.
I'd heard Tim Keller speak on the subject before, which was partly why I bought the book. And while his book was really good, I felt a bit cheated because it is very short, and having double-spaced lines didn't help. Keller's exegesis and application is brilliant, of course. But it was Nouwen who made me cry.
Nouwen misses a few of the theological points, but he feels the whole thing so clearly. Nouwen starts with both the passage and Rembrandt's famous painting of the Return of the Prodigal Son, interweaving what he says with both his own and Rembrandt's lives.
Keller is brilliant, but Nouwen really shows the power of testimony to encounters with God. Read both.