The Prince by Machiavelli is a classic of political science. It is probably the first piece of political theory written from a political perspective that we could fully comprehend. Machiavelli sets up his book in a question-and-answer format. He presents a certain issue, whether it be the use of mercenaries or the appearance of virtue, and then gives you an answer backed by history. He almost always uses an example from antiquity and one from contemporary Italian affairs to support his point.
The most famous point from the book is the often quoted "It is better to be feared than loved." However, the actual quote is closer to "It is best to be equally feared and loved, but since it is difficult to combine those in one person, it is safer to be feared than loved." This gives an interesting insight into Machiavelli's political philosophy. He is ever practical: his suggestions have nothing to do what is right or good and everything to do with what works.
Machiavelli can be credited (or blamed, depending on how you look at it) for the development of modern materialism. Not materialism of our commercial understanding, a desire for stuff, but a materialism of only dealing with what is real. Machiavelli didn't concern himself with political ideals. All that mattered was luck and skill and power. He wasn't immoral, he was amoral. Morality wasn't the aim of his politics, as it had been (at least ostentatiously) since Aristotle. He was concerned with how one attained and maintained power. Our modern understanding of politics stems from his materialism, whether you like it or not.