If there is one self-help book that is really good for fitness professionals to read it is The One Thing by Gary Keller. A few years ago it was a lack of information that held people back. Today it is too much information that gets people in trouble. Today there is so much information that fitness professionals want to try but they get overwhelmed, overthink things and don't take action as a result.
This book does a great job of breaking things down into small chunks that people can take action on right away.