I put The Prince on my Classics Club list mostly out of curiosity. Now that my curiosity has been satisfied, I can well understand some of the controversy surrounding it.
Machiavelli wrote the book for the newly arisen Medici family, and the last chapter is basically a plea for Lorenzo di Medici to rise up and conquer Italy. The Prince is a treatise on power: how to get it, how to keep it, what to do with it. It is utilitarian rather than moral. For example, it advises princes that they need not honor their promises once they are in a position of power if the promises are not in their best interests.
Although Cesare Borgia was considered ruthless and cruel even in his own time, Machiavelli several times holds him up as a model and clearly venerates him. But then, his ideas are not ours, for he tells a story of a principality being won. The principality needed good government, so the prince put in charge a man known for his ruthlessness and rapacity. Once the area was settled, the prince “wiped out” his lieutenant. Good work!
The book is regarded as a realistic analysis of the pursuit of power. This is why it is still widely studied. It is written in a straightforward style, assertion followed by example.