Marx read Proudhon's book late in 1846 and responded strongly and negatively, authoring a lengthy letter to his Russian correspondent P.V. Annenkov on December 28, 1846 with a detailed exposition of his views that became the core of his 1847 book.[11] He began working on a book-length formal reply the following January, completing the work in the spring and going to press in April 1847.[11]