Covey, S. R. (1991). Principle-Centred Leadership. Pocket Books.
Lua i Stíl Chicago (17ú heag.)Covey, Stephen R. Principle-Centred Leadership. London: Pocket Books, 1991.
Lua MLA (9ú heag.)Covey, Stephen R. Principle-Centred Leadership. Pocket Books, 1991.
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