Discorsi sopra la prima deca di Tito Livio
Niccolò Machiavelli is primarily known as the author of The Prince. His Discorsi sopra la prima deca di Tito Livio (Discourses on Livy, written in 1513-1517 and published posthumously in 1531) is in many ways a different work, although how different it actually is has been a matter of dispute. In the second chapter of The Prince Machiavelli himself makes a distinction between the books in terms of subject matter: while The Prince is about principalities, the Discourses is about republics.
Summary
If The Prince resembles a guidebook based primarily on empirical observations, Machiavelli wrote the Discourses as a commentary on Livy's work on Roman history. However, both books include empirical observations and historical generalizations. Machiavelli himself does not make a sharp distinction between the two methods of inquiry, as he thinks that all ages are fundamentally similar. He thinks we can use both methods to teach ourselves the unchanging laws of the political universe. When we have understood these laws, we can use our understanding in political life to achieve our goals.
The book is strictly speaking three books in one. In Book I Machiavelli focuses on the internal structure of the republic. Book II is about matters of warfare. Book III is perhaps most similar to the teachings of The Prince, as it concerns individual leadership. The three books combined provide guidance to those trying to establish or reform a republic.
Reaction
Francesco Guicciardini, Machiavelli's friend, read the book and wrote critical notes (Considerazioni) on many of the chapters.