The first book of modern political theory, Machiavelli's Prince, is a must read. He is the first philosopher to objectively look at the effectiveness of politicians rather than their justness. If you are a careful reader Machiavelli will also keep you well entertained as he relates anecdotes of princes of the past. The prince is also very relevant today for it explains so much of modern day politics.

Read it! You will not regret it!