The article "Energy Efficiency: The Definitive Guide to the Cheapest, Cleanest, Fastest Source of Energy" by Steven Fawkes is a comprehensive guide for managers and policymakers on energy efficiency. It covers what energy efficiency means, its potential, importance, management processes, useful technologies, policies, financing, and engagement with energy suppliers. The author discusses barriers and opportunities for improving energy efficiency, including untapped potential, effective energy management, existing technology, rebound effects, and innovative financing options. This guide is essential for anyone interested in reducing energy supply and demand.