Peter F. Drucker (2014). “The Peter Drucker Collection on Managing in Turbulent Times: Management: Revised Edition, Management Challenges for the 21st Century, Managing in Turbulent Times, and The Practice of Management”, p.43, Harper Collins
Peter Drucker, Alan Kantrow, Rick Wartzman, Julia Kirby (2016). “Get the Right Things Done: The Drucker Collection (6 Items)”, p.12, Harvard Business Review Press