Habits are an interesting beast. They allow us to figure out strategies for our clients and to understand why one person can hang onto every word you say versus only being able to stick to a program for a short period of time.
The following books look into human behavior and how we can motivate ourselves.
Gretchen Rubin's Better Than Before delves into human nature. This link will take you to a blog post/questionnaire regarding what kind of person are you regarding habits.
Darya Pino Rose, Ph.D. is a neuroscientist and former dieter. Her book, Foodist and blog: http://summertomato.com discuss human behaviors and how to analyze your own individual eating habits and situations.