Always Ready
This is Bahnsen's most basic book. It was originally a series of articles Westminster Seminary. It covers all the basics.
Van Til's Readings and Analysis
This is the Bahnsen's magnum opus, the highlight of his apologetics. It is much more thorough (and sometimes hard to read) than A R.
Apologetics to the Glory of God
John Frame alternates between amazingly clear and "do what?" Nevertheless, I would read this one first.