Douglas Groothuis (PhD and a BS from the University of Oregon, and an MA in philosophy from the University of Wisconsin–Madison) is a well-known, respected Christian philosopher and apologist at Denver Seminary and visiting instructor in apologetics for Westminster Theological Seminary. If I had to recommend just one reference book on Apologetics, it would be his 800+page, classic magnum opus “Christian Apologetics.” Make sure you get the Second Edition, which has expanded material.