Thanks to everyone who has chimed in on the question of whether there's a true successor to Mere Christianity. The discussion has prompted many thoughts, but I'll mention three for now:
1) Several people (both in the comments and in emails) asked whether we really need a new Mere Christianity. In a sense, I think the answer is no. Like Augustine's Confessions, Mere Christianity is unique; perhaps we should simply be grateful for the gift. But I also like to think that each generation has a classic that has permanent value yet also speaks to that particular generation. I don't feel as though our generation's classic has emerged yet.
2) I especially appreciated all the suggested readings. A few I've read, but others I haven't and now plan to: Michael Green, Rav Zacharias, even D'Souza. Several are on my desk waiting to be read (Francis Collins, Phillip Yancy). I have read Pascal (with a friend) and loved a number of his pensees, but they struck me more as brilliant isolated insights than as a complete apologetics.
3) Somewhat related to the comments by Bill and others that we no longer seem to have a common culture, it may be that different books will appeal to different kinds of people. Tim Keller's book will appeal to people who worry that the new atheists have made an irrefutable case against Christianity, Collins to those who are particularly concerned about Christianity and science, Chuck Colson to those who are likely to be moved by a powerful conversion story, and so on. Perhaps at this particular moment, the best strategy is to give our friends two books: Mere Christianity and whatever other book seems to fit best.