This is a great place to start in understanding a distinctly Reformed presuppositional approach to Christian apologetics. Van Til stresses the antithesis between Christian and non-Christian thought and the myth of religious and philosophical neutrality. In his understanding we are either for God or against him. Therefore when Christians defend their faith they ought not to treat those they are in dialogue with as neutral, impartial or truly objective. While the use of evidence is not ruled out by any means Van Til is clear there are are no such things as brute or uninterpreted facts.
On rationality Van Til stresses that without the truth of the infinite personal Triune God of Scripture we have no way of knowing anything truly. But God, who knows all things, has been pleased to share a part of his infinite and exhaustive knowledge with human beings.
On Scripture, Van Til's position is that God's verbal revelation of himself in the Bible is self-authenticating and ultimately cannot be made subject to autonomous human rationality precisely because human autonomy is carried out in open rebellion to the God of the Bible.
Christians who favour more classical approaches to defending the faith (such as the traditional proofs for God's existense) would do well to consider Van Til's more biblical apologetic. He begins the task of defending the faith once the content of the faith has been established. If this seems like arguing in a circle then it is also true for Van Til that all people argue in circles. The revelation of God is the ultimate presupposition and court of appeal for Van Til because our ultimate authority (revelation, empirical evidence, reason etc) will always be part of our arguement. In this he is critical of some of his predecessors in the Reformed tradition (notably B.B. Warfield).
Simply a great stimulating read from perhaps the most significant thinker in 20th Century Christian apologetics.