Clark, Fred E.
Macmillan, 1933
Buku Teks SO
Frederick A. Praeger, 1966
Buku Teks SO
Rand McNally, [Date of publication not identified]
Buku Referensi
edited by. Valters Nollendorfs and Geoffrey S. Koby
German Studies Information, 1991
Buku Referensi
Grolier Incorporated, 1964
Buku Referensi