The Prince. The Ruthlessly Honest Handbook on How Power Is Actually Won and Kept

Written as a job application to the Medici family, Machiavelli's short treatise on power strips away every polite fiction about how rulers actually govern - is it better to be loved or feared, when should a promise be broken, what does a leader owe the people versus the state. Five hundred years of politicians, executives, and strategists have read it as either a cynical instruction manual or the first honest book ever written about power.
It is probably both. One of the most influential and most misquoted books in political history - still assigned in political science courses, and still argued about in exactly the terms Machiavelli set.