This is probably the best book I've read to understand postmodernism and it's imact upon the church. Grenz is highly readable and immensely insightful in making plain the complexities and realities of our day.
Is a good book to read if you're searching to (and, I know this is ironic) nail down postmodernism to a few tenets you can understand. Provides the necessary framework to understand the topic when it comes up-- and, at the same time, Grenz does a good job of sifting out the negatives of the movement while pointing to the opporutunties it holds for Christianity to communicate to this culture.
Does not contain much info at all an literature, art, and architecture, which are the main purveyors of the movement. Then, again, by the time most of us in the Church catch wind of anything like PM it's already in the rear-view mirror for most of society, anyway.