Beginning with the first paragragh, this 1970 English translation of the famous 1840 book by Pierre-Joseph Proudhon answers the question posed in the title. "What is Property? It is theft." This is the most famous quote associated with Proudhon and this book is the original source of that quote.
If there is one book of Proudhon that is read most often it is this book. However, the book was written by Proudhon early in his literary career and some of his later writing depart from the early radicalism portrayed in this book.
Consequently, the student of Proudhon seeking to understand the totality of the Proudhon's life and writings will have to read some of his later and less well-known writings in addition to this work.