Books on population or evolutionary genetics?
TL;DR
I'd recommend Population Genetics: A Concise Guide (Gillespie) for an introduction to population/evolutionary genetics (thanks AGS for highlighting this big miss of mine on the first version of my answer).
I'd recommend A Biologist's guide to mathematical modelling in evolution and ecology (Otto and Day) if you want to ensure your knowledge in mathematics by learning their application to evolutionary biology.
General Entry Books to Population Genetics
There are several books that offer an introduction to population genetics. I read Principles of Population Genetics (Hartl &amp; Clarke). I appreciated it but if I were you I think I would rather try Elements of population genetics (Charlesworth) or Population Genetics (Hamilton). There is also Genetics of Populations (Hedrick). I would tend to think that this last book presents lots of empirical population genetics data and doesn't take as much focus as the others in theoretical concepts (but I might be wrong). Gillespie's book Population Genetics: A Concise Guide is a classic. It is short, very easy to follow and pleasant to read. Gillespie's book might eventually be a little bit outdated but I would still highly recommend it.
Emphasis on Analytical Modelling
A Biologist's guide to mathematical modelling in evolution and ecology (Otto and Day) is a very good and very accessible book. It makes a good review about all subjects that are usually taught to first year students in Biology such as linear algebra for example. It is highly accessible and in the meantime it goes pretty far as it ends up talking about the application of diffusion equation in population genetics (Kimura's work among others). This book presents some important models in population genetics but as it aims to provide the tools for mathematical modelling in ecology and evolution it may under-considerate some fields of ecology and evolutionary biology. For example, the book does not talk about population structure nor about evolutionary game theory and there is little about Coalescent theory.
Other books treating specific subjects within population genetics
Coalescent Theory: an Introduction (John Wakeley) is a good book. I haven't read it completely for both time issues and because the math are a bit complicated for me. Coalescent theory offers a very important set of mathematical tools in evolutionary biology.
There's also Mathematical population genetics (Ewens). I am currently reading it. It is definitely not an introductory book and it really doesn't cover much of the most common fields in population genetics.
If you are particularly interested in age-structured population, Evolution in age-structured populations (Charlesworth) is a very good book.
Modeling evolution (Roff) offers some discussion on how to mathematically define fitness from phenotypic traits. While it is interesting I would not counsel you to buy it. Moreover, all the mathematics are quite basic and it aims to explain how to perform mathematical modelling with R which is to my opinion not essential to learn as other languages make a better job at dealing with math (Mathematica for example).
Evolutionary Conservation Biology (Ferrière, Couvet and Dieckmann) is a very good book of conservation and conservation genetics. It develop some mathematical models that are of special interest to conservation of populations and communities.
Ecology, Genetics and Evolution in Metapopulations (Hanski and Gaggiotti) is a book that may interest you as well. However it focuses much more on ecology than the other ones I cited above. Note: I haven't read it entirely.
If you are interested in kin selection and level of selection, you might want to have a look at Major Transitions In Evolution (Maynard Smith and Szathmary) which is a classic (I have not read it though) or The Major Transitions in Evolution Revisited (Calcott and Sterelny) who encompass the opinion of many authors on the subject. Evolutionary Dynamics: Exploring the Equations of Life (Nowak) is also of interest esp. for those interested in epidemiology. Finally, you might want to have a look at the extremely well written and easy to read books by Dawkins such as The extended phenotype (Dawkins) for example. The Dawkins book are very popular and very very introductory. It offers more a way to think than the actual science behind evolutionary biology.