I bought this book with no expectations. I had read an article about it, as an atheist I liked the idea and wanted to support Grayling's work so I bought it. I really was pleasantly surprised.
This book is not a compilation of work. You will not open it up and find passages from various works called out by author. It is truly written in the style of the Bible - Grayling has taken the collected wisdom of hundreds of secular philosophers and melded it together in the flowery prose form typical of the Bible. There is no reference list at the back to tell you where anything came from, he has taken the ideas and the texts and melted them together.
The Good Book begins with Genesis, where you can see the ideas of Darwin laid out in an inspirational way. Reading through this I was really pleasantly surprised at how much I liked it. I'm used to this language being peppered with things I don't believe in, and listening to it at weddings or funerals generally gives me a lump in my stomach. Here was the same kind of lofty language, but saying the things I believed! I didn't realize how much I would enjoy hearing Darwin's theory of evolution told as a beautiful story of how we began and the cycle of life.
As you make your way though the book, you can clearly see where he has incorporated Plato's dialogs, but without specific references. Characters are mentioned in the same way as the Bible, introduced without preamble, just snippets of conversation or story which attempt to showcase an idea. There are passages on grief and death as well.
If you are looking to learn the works of these philosophers in an intellectual way, this is not the book for you. This book is meant to absorb the ideas they upheld in a more spiritual feeling way. I wish this book had been published before I got married, I would have looked for a passage from it to read at my wedding. I could see reading it at a funeral, there really is something cathartic about having grief and death and moving on with life written in this way.
I feel in some ways as an Atheist this may have been what I was missing and didn't realize it. I didn't have anything to help me find a way to feel good about my place in nature's greater story, I just felt the absence of the belief in an afterlife. I also feel like this is a book I could read to my kids to help them figure out how to be a good person in a world which is not always good and in which you cannot rely on a all powerful being to save you from your problems. The language is flowery and poetic, but its also much more accessible than say Kant's Metaphysics of Morals. I minored in philosophy in college, I'm by no means an expert, but I have read some of these works and they are by no means easy to absorb.
So, I think for what this book is trying to accomplish, what it says it is in the description, it is a great work.