The Besancon archive contains a number of Proudhon's manuscripts, but also several scanned books, one of which appears to be quite obscure:
That's roughly "How Business Goes in France, and Why We Have War, If We Have It: Regarding Some New Plans for Agreements between the Railroad Companies and the State." Unsurprisingly, given the title, some of the contents resemble Proudhon's other writings on the railroads, while others are obviously an anticipation of War and Peace, which he would publish in 1861.
It's really strange to run across a book-length work by Proudhon which I don't recognize. But this one seems quite obscure. Proudhon mentioned it in one letter, as it was about to be published. Darimon mentioned it in passing as well. One of the very few scholarly mentions I've found so far calls the published version a "partial" version of an existing manuscript.
I've been reading bits and pieces of this, and translating the section on the "General Theory of War." Like a number of the more obscure volumes, it is a bit of a mixed bag, but parts of it are very interesting.