Origin of Species
This is an edited version of the original flagship work published by Charles Darwin that officially represents the treatise on his theory of natural selection. The ideas presented in this work are responsible for founding what is now known as the theory of Darwinian evolution. Not very technical in nature, but very well written. Much of what this work lacks is owing to the limitations of scientific instrumentation and the refinement of scientific discovery. This is a very important work to read for anyone who wishes to understand exactly what Darwin taught. Primary sources are always important to any discussion concerning their influence. Recommended for reference and familiarity, especially for the more serious student.
Darwin, Charles. The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. New York: Gramercy Books, 1979 (orig. 1859).