The best political science books answer two different questions: what makes power legitimate, and how states, parties and voters really behave in practice. Deciding which of those two you actually want is the real choice, because a founding text and a modern case study leave a reader with very different equipment.
How Democracies Die is the best first book here for most people, current comparative research written for readers outside the field and clear enough to argue with by chapter three. Readers who would rather begin at the source should take The Prince instead, which is short, blunt and behind almost everything written since.
Political science sorts itself by method more than by topic: some books build an argument from first principles, others test one against a stack of countries. Both kinds are here, along with two that ask who holds authority once the formal rules have been written down.