The Prince is a treatise written by Niccolò Machiavelli and published in 1532. Initially intended for Lorenzo di Piero de' Medici, who briefly ruled Florence from 1516 until his death in 1519, The Prince was distributed to some extent in 1513 as well, years before its formal publication.
Machiavelli's treatise serves as a guide on how to be an effective prince, and it covers a lot over its twenty-six chapters (though the book itself is quite slim). Considered one of the first works of modern political philosophy, it deals with such matters as cruelty vs. mercy, the various types of princes (as in both how they become rulers and how they choose to rule), how to deal with common folk, flatterers, militias, fellow princes, etc., and how to apply "all the fierceness of the lion and all the craft of the fox."
Machiavelli also provides plenty of historical examples of good and bad leaders and, with his infamous brand of "Machiavellian" detachment, he describes with remarkable eloquence how a prince can bend others to his will, and recommends crushing them should bending prove difficult or futile.
For me it was an interesting read, but I would have gotten more out of it had I known more about the princes he refers to or, I suppose, if I were a prince myself, as it's basically a "you should do this in such and such a case" guidebook for such leaders, particularly those in the job back then. Apparently The Prince came to have a tremendous influence on politics over the centuries, and for the present-day reader it contains interesting uses of language such as some wildly inventive metaphors and similes. Also, if you're a Game of Thrones fan, you might find yourself repeatedly trying to categorize George R. R. Martin's characters according to Machiavelli's descriptions.