The article deals with the epistolary heritage of Nikolaj Ivanovich Sieber (1844-1888), who was a representative of the classical political economy and a follower of Ricardo and Marx. Since Sieber's personal archive has not survived, the legacy is presented in twentytwo letters, which he addressed mainly from Europe to Pyotr Lavrov, Rosalia Idelson, Pyotr Kropotkin, Pavel Axelrod, and to his closest friends and colleagues-Alexander Chuprov and Ivan Yanzhul. Collected from the archives of St. Petersburg, Moscow and Amsterdam, Sieber's letters shed light on the circle of his correspondents, the nature of relations with publishers and editors, and clarify a number of issues of his theoretical legacy. In the context of the debate about the nature of Sieber's Marxism, the letters reinforce the position that he was a consistent theoretical Marxist and adherent of classical political economy. The details connected with the publication of the book «Essays on the Primitive Economic Culture» are extremely important. For researchers, it remained not entirely clear why Sieber never introduces the concept of «economic culture» and practically did not use it in his work. Correspondence with Yanzhul made it clear that the author proposed a different title-«Economics and Law of Primitive Society». This title clearly corresponds to the logic of the book, since it focuses primarily on the comparison of two spheres of the evolution of primitive, pre-state society strictly interconnected: economic institutions and legal relations. In general, the letters of Sieber indirectly confirmed the thesis that he was a consistent Marxist before the appearance of Marxism and an economist-anthropologist before the advent of economic anthropology.