Many popular diets are commercialized, meaning that the creators make money promoting and selling cookbooks, apps, meal trackers, and other products to help people follow the diet. Once you've selected a diet to research, check to see if that diet has a dedicated website (there are some examples below). This is a great place to begin your research! Generally these websites try to portray their product -the diet- in the best light possible...after all, they want everyone to buy in! Gather as much information as you can from the website, then expand your research to other, more objective sources.
A Bone to Pick by 
The Case Against Sugar by 
Human Nutrition by 
Vitamania: Our Obsessive Quest for Nutritional Perfection by