It's important to note that the Pareto Principle is NOT some kind of scientific law, but rather a rule-of-thumb principle that is easy to misuse. It was not developed by famous economist Pareto. It makes intuitive sense, but doesn't necessarily apply in all situations.
I point this out because the real danger I see in self-help phenomenon like this is when we start to treat them like scientific law. What the authors offer is that this is a way of looking at life that helped them, and might help you. Generally this method is based on some form of lay (i.e. not scientific) psychology; the best add in observations from decades of personal coaching. The worst are selling snake oil, knowing that offering people an easy solution to life is an great way to sell books, whether their solutions work or not.
So I say take these with a big 'ol grain of salt. Which is not to say that they are useless. David Allen's
Getting Things Done system has done a lot for me and helped my gain a more productive view of what work is. That said, I quickly realized that blindly implementing his system would do more harm than good, because it would trick me into believing I'd done everything I needed to live a happy, productive life. Life isn't that easy.