A good book to get the basic ideas of population genetics. The explanations are fairly good though there are a few that require some additional thought or research before they make sense. If you are looking to get an understanding of the many theories and models that are used, this is an excellent volume that gives short, but complete descriptions.
This volume will require some facility with calculus I like manipulation (that is, you should be ready for a lot of algebra and have an awareness of what integrals mean).