Matthew Pierce suggests The Varieties of Religious Experience by William James. This classic, first published in 1929, is available in reprint. James spent his entire career as a Professor of Psychology at Harvard. This work focuses on Religious Experience as an argument of the existence of God.
I suggest The God Question by J.P. Moreland. Moreland, Professor of Philosophy at Talbot Seminary. This book is a new release, and is helpful because of its Christian reflection on recent publications concerning Intelligent Design.
Matthew also recommends The Existence of God by Richard Swinburne. Now in reprint, Swinburne argues that the greatest case for the existence of God does not lie in any individual argument, but the collective weight of the philosophical arguments.
Mere Christianity by C.S. Lewis is a classic that every Christian ought to own in their personal library. Written a half century ago, Lewis, a former atheist, explains his logic for coming to believe in the existence of God based on the Moral argument.
Matthew’s final pick is There is a God: How the World’s Most Notorious Atheist Changed His Mind by Anthony Flew. Flew, who championed atheism for nearly 50 years, came to the decision that God exists persuaded by the recent developments in DNA research.