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 Chinese oral history project collection (中國口述歷史項目檔案) provides a wealth of information on the development of the project and its interviews with eminent Chinese political figures abroad in the United States and Hong Kong from 1958 to 1980s. The highlights of the collection consist of the administrative subject files, correspondence, interview photographs and reports, transcript drafts, collected autobiographies and manuscripts, audio recordings, and card files of names mentioned in the transcripts.