Higgins, James E., author
Teachers College Press, 1971
Buku Teks
Ann Kumar, author
Fakultas Ilmu Pengetahuan dan Budaya Universitas Indonesia, 2020
Artikel Jurnal
Roojen, Pepin van, author
Thw Pepin Press, 2010
Buku Teks
Tomlinson, Carl M., author
Allyn and Bacon, 2002
Buku Referensi
Knowles, Murray, author
Routledge, 1996
Buku Teks