Tim Keller has released a book entitled King’s Cross and it reads very much like a commentary for the book of Mark. As Keller describes, he chose Mark over other gospels because you get more of Jesus in it. This would be a great book to give to someone who is “kicking the tires” on Christianity. Here’s an excerpt describing the book:
“It is an extended meditation on the historical Christian premise that Jesus’s life, death, and resurrection form the central event of cosmic and human history as well as the central organizing principle of the world—and how we fit into it—is most clearly understood through a careful, direct look at the story of Jesus. My purpose here is to try to show, through his words and actions, how beautifully his life makes sense of ours.”
Here’s a brief review of the book as well.