The Origin of Species
It's hard to talk about The Origin of Species without making statements that seem overwrought and fulsome. But it's true: this is indeed one of the most important and influential books ever written, and it is one of the very few groundbreaking works of science that is truly readable.
To a certain extent it suffers from the Hamlet problem -- it's full of clich's! Or what are now clich's, but which Darwin was the first to pen. Natural selection, variation, the struggle for existence, survival of the fittest: it's all in here.
Darwin's friend and "bulldog" T.H. Huxley said upon reading the Origin, "How extremely stupid of me not to have thought of that." Alfred Russel Wallace had thought of the same theory of evolution Darwin did, but it was Darwin who gathered the mass of supporting evidence -- on domestic animals and plants, on variability, on sexual selection, on dispersal -- that swept most scientists before it. It's hardly necessary to mention that the book is still controversial: Darwin's remark in his conclusion that "Light will be thrown on the origin of man and his history" is surely the pinnacle of British understatement.
Editions (via Amazon):