Since even a used edition of Hartl & Clark's Principles of Population Genetics is kind pricey for some, I found this site titled Wyman Nyquist's Notes on Statistical Genetics, with a focus on Animal and Plant Breeding, basically a compilation of course notes & PDFed portions of a text-book. Haven't read it, but the contents look useful if you want to dig deeper into the field of quantitative population genetics. For a soft-landing intro see my post "quantitative genetics".