Evolution:
And a couple of harder/more technical ones:
The Ancestor's Tale is challenging due to the sheer scope of the thing. I found it absolutely fascinating, but as a non-biologist, there was an awful lot for me to learn.
The Extended Phenotype feels like a more technical companion to The Selfish Gene (and it was written as a sequel to that book). It strikes me more as a book written for other biologists than as one written for the general public, at least by contrast to The Selfish Gene itself. Dawkins is an excellent writer, so it's still quite accessible, but definitely not on the same level of accessibility as much of his other work.