Herbert Spencer's laissez-faire and Social Darwinist views are contained in his The Man Versus the State (1873). Biographies of Darwin include Charles Darwin: The Man and his Influence (1990), Darwin (1991) by Adrian Desmond & James Moore, and Charles Darwin (1995) by Janet Browne.
The amount of literature - hardcore scientific and popular - on the subject of evolution is simply vast...
As well as producing books on popular genetics, such as In the Blood, (1996) Steve Jones has recently written an 'update' of Origin - Almost Like a Whale (1999) - which addresses that which has been learned about evolution since Darwin.
Richard Dawkins has produced a range of books on the subject of evolution - the most famous remaining The Selfish Gene. (1976, 1989) See also The Extended Phenotype (1982), The Blind Watchmaker (1986), A River out of Eden (1995), Climbing Mount Improbable (1996), and Unweaving the Rainbow (in which Dawkins addresses the perception of the world through the understanding of scientific theories on a range of topics, maintaining, like Darwin that there is "grandeur in this view.")
Dawkin's main populist opponent in terms of viewpoints on evolution is Steven J. Gould, whose collections of essays provide accessible and often novel takes on evolution - for example The Flamingo's Smile (1987) Bully for Brontosaurus (1992), Eight Little Piggies (1994). Gould's book Wonderful Life contains an excellent introduction concerning the nature of speciation and evolutionary change.
The topic of sexual selection is covered in Matt Ridley's The Red Queen. Ridley's excellent The Origins of Virtue (1996) deals with many issues of evolution that relate to animal and human behaviour. E.O. Wilson's Sociobiology: The New Synthesis (1975) remains an excellent introduction to the topic in general.
Much fiction since Origin cannot have helped but pick up on the ideas generated by the ongoing debate surrounding Darwin's theories. One of the first author to specifically engage with Darwin's ideas was H.G. Wells - see for example The Time Machine (1895) and The War of the Worlds, (1898) - and the baton has been carried by much science-fiction and general fiction since. Today such authors as Ian McEwan - see Enduring Love - seek to continue a reconciliation of the biological sciences and literature.