Bear, Mariah
Berkeley McGraw-Hill 2000,
Buku Referensi
Clinton, Chelsea
Listening Library , 2015
Buku Teks
Terwelp, Wendy
American Society for Training & Development, 2011
eBooks
Scott, Gini Graham
American Management Association, 2009
eBooks
Garbutt, Douglas
Pustaka Binaman Pressindo, 1994
Buku Referensi