Neijt, Anneke
Foris Publications, 1990
Buku Teks
edited by Istvan Batori, Winfried Lenders, Wolfgang Putschke
Walter de Gruyter, 1989
Buku Referensi
Boot, M.
Servire Katwijk , 1984
Buku Teks
Longman , 1995
Buku Teks
Hays, David G.
American Elsevier puvlishing, 1967
Buku Teks