I have an idea about starting a book and publishing it to kindle. It's about a diet style that I have great experiance with it and also transformed my self with it. Losed 35kg. I am thinking about it but there are a lot of books about this diet. Does that mean that my book isnt going to be competive? How can i check if the niche i decide write the book for, is going to be good enought for me to sell some book copies?