Huebner, S. S., author
Appleton Century Crofts, 1972
Buku Teks
Dougharty, Harold
Buckley Press Ltd. , 1949
Buku Teks
Trieschmann, James S.
South - Western college Publishing, 1995
Buku Teks
Magee, John H.
Richard D. Irwin, 1951
Buku Teks
R & R Newkirk/Longman, 1985
Buku Referensi