American College of Sports Medicine 
ACSM’s Behavioral Aspects of Physical Activity and Exercise [PDF ebook] 

Based on the latest scientific research findings, ACSM’s Behavioral Aspects of Physical Activity and Exercise lays the theoretical foundation of behavior change and then provides specific strategies, tools, and methods to motivate and inspire clients to be active, exercise, and stay healthy. Developed by the American College of Sports Medicine (ACSM) and written by a team of leading experts in exercise science and motivation, this highly practical book provides step-by-step instructions to help fitness/health professionals and students master motivation techniques that have proven successful in helping clients adhere to an exercise program.