Probably too similar, but do follow up with "The Extended Phenotype" if you haven't already.
The importance of The Selfish Gene is to write a correct summary of the neo-Darwinian project. In the future (perhaps even now), it will be more famous for the coining of meme (in an appendix IIRC).
However, The Extended Phenotype is a remarkably original exposition of important (then) contemporary frontiers of Darwinism. It is a powerful idea, powerfully described.
It is certainly his most important original contribution to the literature. And a beautiful book. True and convincing. It will change your view of the world. There is no higher praise.