I'm going to be a little radical here and suggest that questions of absolute truth are probably not the best place to start with a post-modern.
The liberal theologian Paul Tillich once put it that, "the fundamental concern of the classical civilisations was about the Fates and the fear of death. The fundamental concern of the Middle Ages was about moral guilt. But now, the fundamental question is not about guilt but about meaning". I would add to that, that because of the rise of science in the eighteenth century the fundamental question at that time was about Truth - was it to be found in all the science of the time, in the Marxist view of history, or where?
Now, please note how the way the church missioned into those fundamental concerns. To the fear of death and the fates the Church preached the Resurrection and the sovereignty of God. To the mediaeval concerns about guilt, Luther preached justification by faith. To moderity's concern about truth we developed all the apologetics ministries. But now?
The British Alpha course has as its tag-line, "an opportunity to explore the meaning of life". Very simply, those are the questions that the post-moderns are asking. Not the questions about guilt or truth that have served as the focus for our mission to date. So that's where you should start. Ecclesiastes is a good part of the Bible for them ("everything is meaningless!"). In terms of reading, try anything from Francis Schaeffer - his stuff from the late sixties is even more relevant now than then. Once they have begun to explore what the Bible says about meaning, the other questions will fall into place easily enough.