Self-help books are usually out there to make money, rather than to inform. Textbooks or published research would have more up-to-date and accepted knowledge.

I'd caution trying to get super knowledgeable about something you don't need to as well. Some things, like eating healthy, are actually quite simple and would take a few sentences to explain about counting calories, and hitting macros. What use would more information be after that? You follow the basics, you're healthy. Unless you're trying to apply this in a professional setting where it's part of your job to stay up to date on the most recent research, you might be using these books as a way to feel like you're making progress on a goal.

Similar for fitness, there's really some basic advice you can get from a place like the CDC or WHO on exercise.

Psychology really can be a minefield - you aren't out there getting your doctorate, you aren't going to be practicing, so what are you trying to learn? How to make friends?

So yeah, while research and textbooks will provide up to date, or comprehensive knowledge, be aware you might not even need that much. And if you do need that much, well, you should already have those resources available from your university or employer. Other than that, self-help is helping their self to your money with repackaged basic info you can find online.