The book includes the fundamental works of the famous Russian and Ukrainian literary critic, academician of the Academy of Sciences of the Ukrainian SSR N.K. Gudzia (1887-1965) on the literature of Kievan Rus in its relations with other Slavic literatures, on the "Word on Igor's Regiment," on Ukrainian-Russian literary unity of the XVII-XVIII centuries, which have not lost their scientific significance and are the basis for further studies of the history of ancient Russian and Ukrainian literature.