Potapova, Nina F., author
Progress Publishers, [196-]
Buku Teks
Katzner, Kenneth, author
John Wiley & Sons, 1994
Buku Referensi
Young, Robert, 1950-, author
John Wiley & Sons, 2015
Buku Teks
Williamson, J.A.
Macmillan, 1965
Buku Teks
Todorova, Elissaveta
Sofia press, 1987
Buku Teks