Dig deeply into some area, become a professional.
Then hire other people and try to optimize your business to work without your control. It's very important to be able to manage the company remotely.
A lot of people are trying to get to some huge expertise to achieve "bigger salary" or something. But no one really focuses on earning a small regular no-hassle income.
That's mostly it...Focus on 20% of stuff which gives 80% of results, like the book said.