A list compiled by the fantastic Martin Willet (www.mwillet.org - be glad this guy is on our side).
I haven't made it all the way through that list yet, but the following I consider to be the most important (so far):
The Origin of Species
(on The Origin of Species by means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.)
Charles Darwin, M.A.
The Selfish Gene
River out of Eden
The Meme Machine