Bauer, Ronald C., author
Teachers College, Columbia University, 1955
Buku Teks
edited by Steffen Bangsow
Springer, 2012
eBooks
Hill, C. P., author
Unesco, 1953
Buku Teks
Scarpe, N. V., author
Unesco, 1951
Buku Referensi
Haas, Kenneth B.A.O.
Prentice-Hall, 1960
Buku Teks