I am partial to the International Society of Sports Nutrition...now mind you this is sports nutrition not clinical. Their CEO Dr Jose Antonio is a brilliant guy, and their 650+ page book, Essentials of Sports Nutrition is awesome. Don't listen to people who say screw the certification. It is good to have a goal, to study for the goal, and then to pass it. It will help you stay disciplined and study better, and that is the goal, to learn and to remember. Go for it.
JM