I can't remember how but when I was 16 I came across this book and it changed my life. The title of Dawkins biography is "An appetite for wonder", and this appetite is no where more apparent than in this book (I have read most of his books). It is a wonderful introduction to the theory of evolution by natural (and sexual) selection, behavioral ecology, and the wonders of nature. At the same time it serves as a terrific example of first rate scientific reasoning. The writing is clear and fluid and extremely elegant. In his autobiography Dawkins admits that every sentence has been rewritten multiple times. Those that have survived this selection process really deliver. Every sentence seem to fill a purpose and yet, rarely does one feel that information is in some way lacking. This book, when it came out in the late seventies, influenced the general public and academics alike. It changed how academics thought about genes and evolution, and it introduced the meme, which has subsequently entered our dictionaries.
As I have said elsewhere, this book really is a literary masterpiece. The fact that it also teaches science to the reader is an added benefit that makes this book one of the best and most important ever written.
The book has a very good structure. At no point does it feel as if new concepts are introduced inappropriately. Dawkins begins by slowly and carefully introducing the replicator concept. In the widest sense a replicator is, as the name implies, something that replicates itself. This can be a mineral shape, a computer virus or a molecule such as RNA or DNA. It is inevitable that a replicator that produce more copies or copies that are more durable will become more prominent in the population. And so it is with our genes. The genes that exist in humans that are alive today are descendents of a very long series of genes that outperformed other genes. To achieve this success the genes have used many different tricks. Primary among these is cooperation with other genes to construct vehicles such as a plant or an animal that can both protect the genes and pass them on. Humans are thus "merely" vehicles created by genes for the benefit of genes (though in another sense we are of course much more than that).
Dawkins carefully builds from this starting point and reaches startling conclusions about many different aspects of nature and evolution. Why did sex evolve and why do the different sexes differ to a greater or a lesser extent in different species? Why are males in general more aggressive? Why do we cooperate? Does altruism exist? How did sterile ants evolve? Whatever he is discussing, Dawkins always provides illustrative examples from nature and when he use metaphors he is (unlike many others) always careful to translate those metaphors back into the language of replicators. The Selfish Gene also derives some of its fame from the fact that it introduced the meme concept. A meme, Dawkins suggested is like a gene in that it can replicate itself, typically via language or imitation. Successful memes (think viral youtube clips) will spread throughout population of less successful memes in the same way that successful genes spread, however, for memes the sexual reproduction of its host matters little. Rather, the success of a meme is determined by its ability to make its host share the idea with others. The meme concept is now in most dictionaries.
Throughout the book Dawkins is careful to point out that even though we are products of evolution and as a result have many instincts that are not always very noble, that does not mean that it is in anyway good or moral to follow ones evolutionary inclinations. Indeed if we understand human instincts we may be better able to construct societies that combat our caveman instincts.