Downes, William, author
Cambridge University Press, 2003
eBooks
Malmstrom, Jean, author
Hayden Book Company, 1965
Buku Teks SO
The Hague, 1972
Buku Teks
Fishman, Joshua A., author
Newbury House Publishers, 1972
Buku Teks
edited by Ulrich Ammon, Norbert Dittmar, Klaus J. Mattheier
Walter de Gruyter, 1987
Buku Referensi