Helfert, Erich A., author
Erlangga, 1997
Buku Referensi
Helfert, Erich A., author
McGraw-Hill/Irwin, 2003
Buku Teks
Helfert, Erich A., author
Richard D. Irwin, 1967
Buku Teks
Helfert, Erich A., author
Richard D. Irwin, 1982
Buku Teks
Helfert, Erich A., author
Richard D. Irwin, 1977
Buku Teks