Tagliamonte, Sali A
Cambridge University Press , 2006
Buku Teks
Maguire, Warren, editor
Cambridge University Press, 2011
eBooks
Enideg Nigussie, Ethiopia
[, Springer Science+Business Media], 2012
eBooks
Spring, Joel
McGraw-Hill, 2008
Buku Teks
Americana Corporation, 1980
Buku Referensi