Keller does a really good job of laying out the apologetic for the existence of God.  As a believer you should read this book so you can give an account of what you believe and why (I Pet 3:15).  This is really a book that every non-believer should read, but it’s tough to get someone who likely doesn’t really think too much about things like this to take it on.  Good to have in the library and re-read from time to time.