Covey, S. R. (1991). Principle centered leadership. New York: A Fireside.
Chicago Style aipamenaCovey, Stephen R. Principle Centered Leadership. New York: A Fireside, 1991.
MLA aipamenaCovey, Stephen R. Principle Centered Leadership. New York: A Fireside, 1991.
Kontuz: berrikusi erreferentzia hauek erabili aurretik.