It is not a real "handbook" in the traditional sense of the word (i.e. a handbook used by mixing engineers), but rather conceptual ideas that point out the key elements of mixing for the beginner.
The reality of this book is that the content is based on a series of pretty informative interviews that are transcribed verbatum in 2nd half of the book. The first half of the book is the author's summary of the usable data culled from the interviews, and presented in a organized fashion.
The 6 aspects covered (that comprise a good mix) should certainly help beginners start thinking about mixing in ways that don't ordinarily occur to beginners. In essence, what this book does best is to tap into the experience of very experienced engineers, and then present "what the experts do" in various scenarios.
There are NOT so much specifics as there are general guidlines. It is not so much "step-by-step instructions" or "hard reference" as it is a "starting point." There ARE useful things like where to start the mix from, and tables that show where to find the EQ frequencies that effect different instruments--for example, to bring out the slap sound on a bass guitar. Further guidlines suggest, which effects and settings (in broad terms) to use to emphasize/de emphasize other aspecs.
I'm a beginner, and it has been a good resource for me.