From the father of Positivism, this book explores the true nature of the norm and the role it plays in relation to state and society. Excellent analysis of positivist normativity.

You should also consider reading The Pure Theory of Law, by Kelsen, as well as books by Rawls, Nozick, MacIntyre, and Posner.

This book rediscovers the most important elements of Law, provinding Neo-Kantian theory aplied to Law and State. Altough, it is one of the most important law books written in the 20th Century, one must prior have read "The Pure Theory of law" in order to come about to a thorough understanding of this manificent pice of art. It has been heavilly criticized by those who follow "The Naturist Theory" and mostly by dogmatic waves, including the Catholic Church, for it proves that relativism refers to Democracy and Republic, and dogmaticism referes to Dictatorship.