Unfortunately I don’t think popularity is a very good metric in this case, there are a lot of wildly popular books that are full of bullshit. Because there is just so much research out there in the world, you can find studies that support pretty much anything you want to promote. That doesn’t mean they’re good studies: A book can just say ‘Studies have found that X is good for you’, and it’s only when you actually look into the studies they cite do you see that it’s just one study with a tiny sample size or no control group etc.

For diet, good studies are generally long term cohort studies - that means they’re tracking the eating habits and health of a very large group of people over a very long period of time, because only then can you really say anything meaningful about long term health effects. Examples are the EPIC Oxford study, Nurse’s health study, and Adventist health study. You can look those up and see what some of their main findings are.

In terms of diet and nutrition, most countries also have a national dietetic association - an association of professionals in the field - and also a government agency that provides nutrition guidelines. While in some countries certain industries have a little too much influence, the guidelines produced by these national associations and government agencies are still mostly pretty good and based on the scientific consensus.

There are several annual rankings of diets by health and sustainability (as in, it’s not hard to keep up long term), and the Mediterranean diet consistently comes out on top.

I don’t know much about where to look for good advice on fitness and psychology I’m afraid.