I flog R. A. Fisher’s The Genetical Theory of Natural Selection a fair amount on this site. You don’t need to understand everything in the book, nor do you have to agree with everything in it, but it is a great point of departure toward understanding evolutionary genetics. I’ve noted that you can get it free in PDF format. But if you want to browse it online in a easier format, here you go:
(original link courtesy of Unz.org)