Health books are about the health and function of the human body. The books in this genre talk about the function of the body and practices, foods, and medicines that improve them.
Fitness books are about exercises and other things one can do to keep their body healthy and fit. There are many books in this genre that focus on how to exercise and train to improve the overall wellness and strength of the human body.
Diet books are about a specific way of eating or selecting food to either improve one's health or lose weight.