I've listed ten of my favorites right here. Let me add to this list David Eller's book, Atheism Advanced: Further Thoughts of a Freethinker, and Jason Long's book, The Religious Condition: Answering And Explaining Christian Reasoning. As you can see, my favorite skeptical books are not scientifically or philosophically related primarily because of what I think about Natural Theology. There are many others. What others are your favorites?