This is the kind of subject where everyone has an (un)informed opinion and belief. Science is non-existent. Millions of "research" reports and articles are out there, and every single one that the Cochrane Collaboration has looked at has been shot down.
Today it's also been infested with "save the world" beliefs, which have nothing to do with nutrition ("meat is bad for the climate").
If you want to inform yourself about the nutritional part, I recommend:
Gary Taubes, "Good Calories, Bad Calories", UK version "The Diet Delusion".
Contrary to most other books, it's not written by someone propagating an own agenda, but a respected scientific journalist.
Put your money where your mouth is, it'll set you back around 20 Euro on Amazon (paperback).