Have you ever been sitting in meditation, and suddenly realized that you don't really know why you're putting yourself through this kind of torture, or at least can't explain why in a way that really makes rational sense? If you have (as I have), you will understand just how valuable a book like this can be to anyone who is just learning how (and why) to be a Zen practitioner.
First, a little personal background:
I grew up in a generically Protestant, nearly-agnostic, nominally Christian setting. I never really had faith in what I was taught, and as I grew older, I evolved from nearly-agnostic to nearly-atheist. I didn't like the idea of completely denying the spirituality that was undeniably a part of so many people's lives, but I couldn't accept the dogma I grew up with, and nobody else's dogma appealed much to me, either.
I started investigating Buddhism for the shallow reasons common to most Western practitioners: I thought the Dalai Lama was cool, Buddha statues were neat, and I liked the artwork I saw. I started reading about the life of the Buddha, and about various schools of Buddhism. It was still a very uncomfortable search for me, though, because despite the fact that it looked like there really was something deeper there, all the talk of "emptiness" and "illusion" seemed more silly than the beliefs I had already rejected.
Then, about 7 years ago, I stumbled across "Understanding Zen".
The book was very easy to read, and presented its philosophical arguments in a style far lighter than most serious Western philosophical texts, but also far more direct and reasoned than most Eastern philosophical texts. It explained what Zen was about in a way that my rational mind could accept, and it allowed me to say "I am a Buddhist" without feeling like I was claiming to believe things I don't believe.
It helped me to grasp on a rational level the idea that thoughts and concepts, even the concept of self, are all simply tools for the conduct of life. That, in turn, helped me release some of my attachment to these concepts, though obviously it is impossible to achieve true enlightened detachment simply by grasping a new concept.
As a result of reading this book, I suddenly had a rational basis I could use to goad myself into sitting and meditating when I didn't want to. I was suddenly able to actually justify to myself, in words, the things I'd been feeling I needed to do.
I know that the philosophical arguments in this book are incomplete and doubtless have many epistemological flaws, but I think it's far better to talk in concrete terms about the difference between the concept of a thing and the thing itself (and the fact that even the concept that it is a thing is arbitrary) than to prattle on about the reflection of the moon on a still pond and tell people to stop looking at your finger.
Either approach may eventually help someone on the path to clearer understanding and even enlightenment, but the advantage of a more rational approach is that people are less likely to go off worshipping the moon and cutting off your fingers when they miss the point.
I found that after reading this book, I was able to approach Koans and more traditional methods of Zen teaching with a clearer mind, and to sit with one fewer concern to disturb my meditation. You probably won't have a moment of Satori while reading this book, but if you're having difficulty reconciling rationalism with Zen, "Understanding Zen" may be a big help.
If you want a book that's super-easy to read, or you can't handle opening up your mind a little, then give up reading about Zen. Otherwise, buy this book!!! So what if some parts have to be read slowly, a few times over? Take your time and you'll be glad that you did. By nature, Zen challenges our old ideas and offers us perspectives that are totally new (and difficult to grasp) for most Westerners. If you only put a little effort into understanding Zen, you'll find that the Radcliffs do an incredible job of explaining the basic (and not-so-basic) concepts.
Obviously, it take a lifetime (or more) to come to "Enlightenment" so don't expect to comfortably wrap your mind around Zen. After reading the book you might understand Zen at least nearly as well as most people who practice Zen meditation, and that's no small accomplishment!
The only drawback to the book is that the Radcliffs fail to answer the question that plagues most people as they start to learn about Zen: "Does Zen contradict my beliefs? Religious or moral beliefs, in particular." Most of the book at least hints at the answer, but the last section of the book just runs away from it. Instead, they delve into highly opinionated, out-of-place, off-topic ranting. Unless you find it interesting, just skip the last section completely. Take from the book what you can and it'll be more than worth your money!
If you are really worried about Zen going against your religion/morals/etc., then here's my quick answer:
Zen doesn't ask you to believe in any thing, nor does it ask you to give up your beliefs or values. It just challenges you to put them into perspective. If anything, I find that Zen gives me more tolerance, patience, and understanding of other people's views. So go ahead and read all you can about Zen. Meditate. You'll be a better person for it.