On the Origin of Species, by Charles Darwin. It's pretty dense, dry, and can require a bit of a commitment, but there's some pretty interesting bits and pieces in there aside from just proposing evolution.
I would also highly recommend the sequel, the Descent of Man. That's where Darwin actually discusses the evolution of humans, a topic he gingerly avoided in TOOS
Great read! I especially like how a lot of his speculation turned out close to what we know today. Choice quote: "I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale."