Books on my shelf right now (I'm an evolutionary genetics grad student)...
Entry level:
Evolution - Mark Ridley (not Matt Ridley)
A primer of ecological genetics - Connor & Hartl
Intermediate:
Elements of evolutionary genetics - Charlesworth & Charlesworth
Advanced:
Principles of Population genetics - Hartl & Clark
Introduction to quantitative genetics - Falconer & Mackay
Further basic/popular-science books include R Dawkins' Selfish Gene/The Extended Phenotype/The Greatest Show on Earth and of course The Origin of Species by Darwin