Tai Chi Qigong begins by covering the basics of qi, or chi - I'll use qi because it's the convention in the book.
I would hazard a guess that most of the first chapter is the same as Dr. Yang's Simple Qigong book. Either one will give you a decent primer on qigong. The only new stuff - for me, at least - in this book's first chapter was covering a brief history of tai chi chuan, or taijiquan.
The key part of this book for me was the second chapter, which talks about yin and yang - the root of tai chi chuan.
Dr. Yang writes that the qigong series in this book are based on the theory of yin and yang: two opposing forces that must balance each other. "If the balance is insignificant, disaster will occur," he says. "However, when these two forces combine and interact with each other smoothly and harmoniously, they manifest power an generate the millions of living things."
By understanding this, he says, you'll have a better idea of what you'll accomplish in your practice.
He discusses still and moving meditation, breathing, mind and movement, and ways to classify tai chi chuan.
The third chapter brings together a number of pictures displaying how to perform the series he mentions. I personally find these chapters less useful than the theory sections since I would much rather learn movements from a live interaction or, at a distant second place, via video. I think there are many intricate movements we miss out on my trying to practice techniques from a book.
Luckily, there is a companion DVD.