- Maxwell, John C. (2007). The Law of Addition. In The 21 Irrefutable Laws of Leadrship: Follow Them and People Will Follow You. Nashville, Tenn: Thomas Nelson.
- Maxwell, John C. (2012). The Law of Contribution. In The 15 Invaluable Laws of Growth: Live Them and Reach Your Potential. New York, NY: Center Street.
- Carnegie, Dale. (1936). How to Win Friends and Influence People. New York, NY: Simon & Schuster.
- Entrepreneur (2014). Richard Branson on Why Making Employees Happy Pays Off. Entrepreneur. Recuperado de http://www.entrepreneur.com/...