My buddy James let me know that he was a little unclear on some of my criticisms of The Reason for God. I went back and read my post through two more times, and James is right -- I don't think I expressed myself very well. Here's another try.
The intended audience for Keller's book is non-Christians. Atheists and agnostics, mostly, but he's writing to followers of other religions as well. The book is laid out into two sections -- the first covers the most common questions posed about Christianity (why does God allow evil, why did Jesus have to die, etc...) and the second half lays out his take on some of the most foundational Christian principles -- grace, love, spiritual disciplines, etc...
So given all of that, I have to think that at some point he wants the reader to ask the question that is asked multiple times in the New Testament: "What must I do to be saved?"
It's the call to action. Every church I've seen has some sort of answer for this question. Some say you must accept Jesus with a statement of faith, others perform baptism, others have a longer-term process with multiple steps of commitment.
Keller doesn't answer the question at all. He spends 300 pages telling non-Christians why they should change their minds, yet if the book actually succeeds in this, he leaves the non-churched reader wondering about the next step. I found this strange.
So this is my main critique with Keller's book:
1) He says that non-Christians are lost and destined for hell (see here on his church website for more on the topic). We'll call this "point A" in a spiritual journey, just for the sake of example.
2) He says that Christianity is not only the key to going to heaven, but it's the key to living the best life on earth. We'll call this point B in the spiritual journey.
3) He says nothing about how to get from point A to point B.
That seemed odd to me.