"What is Property?" by Pierre-Joseph Proudhon is a groundbreaking work that challenged the foundations of 19th-century society. 

First published in 1840, this provocative treatise questions the nature and legitimacy of property, famously declaring "Property is theft!" Proudhon examines the origins and impact of property rights, delving deep into issues of liberty, equality, and justice. His arguments are a foundational text for anarchist philosophy, critiquing the inequalities perpetuated by private property. He explores the idea that true freedom and social justice can only arise from a society where property is not the basis of wealth. His incisive analysis extends to the nature of government, law, and class structure, offering a radical perspective on the societal constructs of his time. 

This book is not just a historical document but continues to be relevant, providing insight into ongoing debates about wealth distribution, social justice, and the role of the state. "What is Property?" remains a vital read for anyone interested in political theory, economics, and the history of anarchist thought.