Papers include Kagan's correspondence, photographs, printed materials and audio tapes. All letters in the collection date from 1938-39, when he had moved his Petropolis operations to Brussels; there are items by Vladislav Khodasevich, Vladimir Nabokov, and Mikhail Osorgin, and others. Among the photographs are pictures of Ivan Bunin, Konstantin Fedin, and Evgeniĭ Zami︠a︡tin. Printed materials consist chiefly of books published by his firms; some of the books are inscribed, for example by Nikolaĭ Berdi︠a︡ev and Lev Karsavin. Included is a lengthy taped interview of Kagan by Marc Raeff in which Kagan discuses his life and publishing activities.

The collection consists primarily of correspondence and manuscripts. There are letters from Korneĭ Chukovskiĭ, Mikhail Karpovich, Andre Mazon, Vladimir Nabokov, and Evgeniĭ Zami︠a︡tin. There is also one item each from Sergeĭ Esenin, Pavel Mili︠u︡kov, Ivan Pavlov, and Nikolaĭ Roerich. Manuscripts include photocopies of poems by Korneĭ Chukovskiĭ, Sergeĭ Esenin, and Boris Pasternak. There are subject files on Dostoevskiĭ, Turgenev, Soviet education, and Slavic studies in the United Studies, and a photograph of Isaak Babel ́with his daughter.