Here is some recommended further reading on Apologetics from Sunday School on August 10, 2008, dealing with chapters 3-5 in Tim Keller's The Reason for God.
Apologetics to the Glory of God: An Introduction by John M. Frame - If you do not have a good book with instructions for presuppositional apologetics then this is where you should start to compliment Pastor Keller's book, which, as I said on Sunday, is really an apologetic rather than a book on apologetics. Professor Frame has a number of arguments in here after giving some opening principles for apologetics that you can try to become familiar with and have in mind to adopt based on specific conversations. Professor Frame is a fantastic writer and makes difficult concepts very clear and understandable for those who are not very familiar with technical philosophical and theological language.
Always Ready: Directions for Defending the Faith by Greg L. Bahnsen - Dr. Bahnsen was probably one of the best apologists that God has yet blessed his church with. This is my favorite book on apologetical instruction. Dr. Bahnsen does not include a lot of specific arguments here but this book will still be helpful for giving Biblical principles for defending the faith and also includes some basic instructions in formal logic.
The Defense of the Faith by Cornelius Van Til, 4th edition ed. by K. Scott Oliphint - This is really the classic treatment of Presuppositional apologetics. This edition includes footnotes by Dr. Oliphint, Professor of Theology and Apologetics at Westminster Theological Seminary. Dr. Van Til's work can be heavy and difficult to read but Dr. Oliphint's notes help to explain some of the terminology and key ideas in Dr. Van Til's thought.
The Battle Belongs to the Lord: The Power of Scripture for Defending Our Faith by K. Scott Oliphint - This book is another basic introduction to presuppositional apologetics. I don't think that it is quite as comprehensive as John Frame's introduction to apologetics but it is very helpful for understanding what Scripture teaches us about why and how to do apologetics. Dr. Oliphint is also a clear writer and you should find this book to have a lot of helpful exposition in it.
Reason and Revelation: New Essays in Reformed Apologetics ed. by K. Scott Oliphint and Lane G. Tipton - This anthology has a number of essays making the case that Scripture is the only foundation for epistemology. These essays tend to be somewhat more technical so it would be helpful to have some familiar with philosophical and theological language about epistemology before starting with this.
The Great Debate: Does God Exist - This is a cd/dvd of 1985 debate between Dr. Greg Bahnsen and Dr. Gordon Stein about the existence of God. This can be very helpful in seeing a more formal debate about Christianity.
Also, here are a few articles online that you can read for free on apologetics and some of the issues that we discussed:
"Evangelism and Apologetics" by Greg Bahnsen
"The Encounter of Jerusalem with Athens" by Greg Bahnsen (an Apendix in Always Ready)
"The Problem of Knowing the 'Super-Natural'" by Greg Bahnsen (also in Always Ready)
"How do we know God?" by Gordon Clark
"Science and Truth" by Gordon Clark
"Why Study Logic?" by John Robbins
"Unbelievers and the Knowledge of God: Biblical Warrant for a Presuppositional Apologetic" by K. Scott Oliphint
"Why I Believe in God" by Cornelius Van Til