Should I never trust in self-help books which involve a financial aspect?

That's good that you're using critical thinking skills and yes you sure can learn from people and their scams. Financial advisors are fulll of sh** too and they all say something different but you sure can learn alot from the small amount of information that they do say that isn't total bullsh**. People are idiots and full of crap, without a doubt so keep being careful and take what info you can that works for you.