Παραπομπή APA

Covey, S. R. (1989). 7 habits of highly effective people.

Παραπομπή Chicago Style

Covey, Stephen R. 7 Habits of Highly Effective People. Pocket Books, London, 1989.

Παραπομπή MLA

Covey, Stephen R. 7 Habits of Highly Effective People. 1989.