I read "The Millionaire Next Door" many years ago; I enjoyed it and learned a lot from it. It was interesting to read how big money earners don't always become millionaires while those who earn less can, and often do, become millionaires. He mentioned school teachers as an example.
Recently, in the news, there was a story about a man who had ordinary jobs like pumping gas and being a janitor. When he died they found he had a net worth of about 6 million dollars. And there are so many stories just like that. So there's no excuse whatsoever for not becoming a multi-millionaire. Anyone can do it if they put their mind to it.
Sorry to hear the news about his passing.