Covey, S. R. (1991). Principle centered leadership. New York: A Fireside.
Dyfyniad Arddull ChicagoCovey, Stephen R. Principle Centered Leadership. New York: A Fireside, 1991.
Dyfyniad MLACovey, Stephen R. Principle Centered Leadership. New York: A Fireside, 1991.
Rhybudd: Mae'n bosib nad yw'r dyfyniadau hyn bob amser yn 100% cywir.