I agree with what you said about main stream fields being diluted, but offer an interesting corollary to that. Economic motives compel various gurus and nutritionists to make claims to the average joe, and the average joe, or even the educated joe cannot sort through them. However, if one looks in more narrow fields, one can obtain more specific answers without so much trash. For example, powerlifting. This is not a huge market nor one you can benefit financially from that much. If one is trying to sell something or get something published, he can't just say "I pretty much agree with X", he needs to somehow distinguish himself. But when that motive is eliminated you can get more consistency in recommendations and have a greater chance to actually hit upon what works.

While you might not be interested in powerlifting, reading in more niche areas can help filter out profit/status seeking charlatans, and can allow one to see the similarities across disciplines. So while I've read about bodybuilding, powerlifting, and endurance sports, and their associated nutritional advice, I would never read a book about "being fit."