I have only ever read one other Dawkins book before, The God Delusion, and really didn't like the style or attitude of the writing, so was not completely looking forward to this one.
The primary aim of the book is to look at all the evidence and theories that make up the Darwinian theory of evolution and natural selection. He considers all the evidence from real life examples, in particular the eye, and buy using a computer program that he wrote, demonstrates how new variants of a species can evolve with very simple initial amendments.
Later on in the book he looks at how DNA works, the effects of positive feedback on evolution and the way that mutations works of evolution and selection. He also considers the complexity of trying to document the tree of life.
Overall I thought this book was a better read than the previous one I had read. Bearing in mind it was written originally in 1986 most of it is still valid, though we now understand far more than we did then. He can get on his high horse quite often, but thankfully not so much in this book.