P. J. Proudhon 
What is Property? [EPUB ebook] 

‘The letters of Proudhon, even outside the circle of his particular friends, will always be of value; we can always learn something from them, and here is the proper place to determine the general character of his correspondence.
‘It has always been large, especially since he became so celebrated; and, to tell the truth, I am persuaded that, in the future, the correspondence of Proudhon will be his principal, vital work, and that most of his books will be only accessory to and corroborative of this. At any rate, his books can be well understood only by the aid of his letters and the continual explanations which he makes to those who consult him in their doubt, and request him to define more clearly his position.’