Good Basic Apologetics
Keller does a good job introducing the subject of Apologetics to the modern church, which unfortunately has pretty much forgotten that aspect of being "Salt and Light" in a Sin darkened world.
The only fault I find with the book is it is basic stuff. But such a book is needed for this generation, as those of Francis Schaeffer were needed for the previous generation.
An idea course of study would be to read Keller's book and then work your way through Schaeffer's first three "The God Who is There," "Escape from Reason" and "He is There and He is not Silent"