Dinesh D'Souza wrote What's So Great About Christianity as a response to today's militant atheists -- you know, the ones who rent billboards proclaiming "There is no God, have a nice life," and publish overhyped books detailing just how much they hate the God they say they don't believe in. This is the same crowd who will doubtless be all over the Pope's pronouncement that human life is not an accident... and that itself is no accident, because it is on the battlefield of evolution that the future of Christianity in the West will be decided. (Whether Western Christians are yet aware of that, let alone prepared for the battle, remains to be seen.)
I heartily recommend D'Souza's book. It does not take an anti-evolution view -- in fact, there are many times when it will seem far too broad to some Christians. But it stands in the tradition of Mere Christianity as one of those books that, while it does not compel conversion, at least renders the modern philosopher without excuse for ignoring the reality of God. D'Souza is a gentlemanly fighter, but he's a fighter and his logic is relentless. His explanations of the philosophical positions of men like Kant and Hume, for example, were the most understandable I've enountered (I may be the child of two philosophy majors, but it generally makes my head ache -- and unfortunately, the knowledge doesn't stick there very well). If you are only familiar with D'Souza from his conservative politics, you should know that this book is largely apolitical and he is careful to maintain a non-combative tone. Except, perhaps, when relaying some of the absurdities of the militant atheists, of which there are quite a few. My personal favorite was the one on p. 161 that I quoted on April 1.
For Christians seeking ammunition against atheistic fundamentalists, this book is ideal. Nonbelievers who just wonder what all the fuss is about, or those who are genuinely seeking and want to know why Christianity is "so great" should read this too -- but for a gentler touch with a little less of the Ivy-League debate format, check out Tim Keller's The Reason for God: Belief in an Age of Skepticism. Keller, as a pastor, avoids pugnacious arguments and succeeds very well in presenting a winsome case for God that will appeal to believer and unbeliever alike. For hard-core apologists who do battle with godless philosophies on a daily basis and have sharpened their minds accordingly, I recommend Nancy Pearcy's Total Truth: Liberating Christianity from its Cultural Captivity. Although it's not the easiest read in the world (plenty of headache-inducing philosophy), it provides an extremely thorough grounding in the idea of "worldview" that is crucial to any meaningful discussion of Christianity and culture.