Everyone knows about On The Origin of Species, which was about natural selection. His second book The Descent of Man, and Selection in Relation to Sex is devoted to sexual selection and it's role in producing things like peacocks...and gender roles in humans. According to wikipedia sexual selection is still debated today, but it seems to have broad support in both biological and social sciences. Further down the rabbit hole he started to write The Expression of the Emotions in Man and Animals before his death.

Perhaps this has all been covered before but I felt it was worth repeating even if it had.