By C.S. Lewis
The book that opens you up to the efficacy of Christianity, not through a tour of scripture, church history or archaeology, but through the unexpected vehicle of logic.
The opening chapters that touch on the Moral Law or Law of Decent Behavior was astonishingly easy to get your head around. Lewis has a way of making sense of things both complex and deep. I could recommend all of his books really. I've also read two of his biographies and most of his other work.
Be sure to check out The Four Loves, The Great Divorce, Surprised by Joy and The Chronicles of Narnia series (You're never too old for fairy tales).