Williamson, H. F., & Smalley, O. A. (1957). Northwestern Mutual Life: A century of trusteeship. Northwestern University Press.
Lua i Stíl Chicago (17ú heag.)Williamson, Harold Francis, agus Orange A. Smalley. Northwestern Mutual Life: A Century of Trusteeship. Evanston, Ill: Northwestern University Press, 1957.
Lua MLA (9ú heag.)Williamson, Harold Francis, agus Orange A. Smalley. Northwestern Mutual Life: A Century of Trusteeship. Northwestern University Press, 1957.
Rabhadh: Seans nach mbeach na luanna seo go hiomlán cruinn i ngach uile chás.