Maintaining efficiency is like maintaining your physical fitness. You have to keep it up. Design your efficiency improvements to survive as long as the facility. Each Measure that requires maintenance tells you how to keep it profitable. Let all your information sources work for you.
Capable professionals depend primarily on a few well-worn references.
But, they also know how to get information from other sources quickly. Whether you are a professional or not, the Energy Efficiency Manual is your primary reference for energy efficiency. However, no single book can tell you everything you need to know. To do battle with energy waste, assemble an armory of information that is appropriate for the level of improvements that you plan to make. You will see that the Energy Efficiency Manual is not cluttered with formulas and tables.