"The Selfish Gene" by Richard Dawkins. It describes biology from the perspective of the gene as the unit of natural selection, rather than the organism or the species, and demonstrates the power of that perspective to explain much about the natural world. But then, the author generalizes the concept of a gene to that of the replicator, which is any kind of pattern that influences its environment to produce copies of the pattern. (As an example, the author invents the concept of a meme, being a unit of culture that uses brains to spread itself across a culture.) This (the replicator) is the mind-blowing concept that I'm still thinking about 35 years after I first read this marvelous little book. Organisms and people and species and cultures are ephemeral side-effects of mindlessly self-replicating patterns. You'll never look at the world the same way after reading this one!