I've never read the book, but my guess is that it mainly preaches live below your means, always try to save/invest, even if it's only a little. Delay gratification as much as possible. Stuff that might seem common sense to those of us in this forum, but as Gumby said, it if set off the light bulb for other people, it's done a valuable public service.
In my case, I remember delaying a bit of gratification back when I graduated college in 1993. The phrase "SUV" hadn't made the mainstream yet, as I recall, but the market for compact SUVs like the Ford Explorer, Nissan Pathfinder, etc, was red hot at the time. I was working "part time" but up to 39 hours per week, for McDonnell-Douglas Space Systems, with the promise of full time in the near future, and thought I was living large on $10/hour. Plus, I had a second job in a department store.
Well, I went through a brief period where I wanted one of those compact SUVs, most likely a Pathfinder. I think the reality sunk in that I'd be paying $25K or more for something that was slower, clumsier, and more cramped than the '68 Dart V-8 I was driving at the time. And, got about the same fuel economy!
If I had gotten myself into that much debt that early on, who knows how it might have made my financial life turn out in later years? Plus, new cars wear out and, eventually, turn into old cars. And once you get used to a new car, I think it's easy to justify on trading on another new car once the current one doesn't feel or look "new" any more. So it could be easy to perpetuate that spending cycle.
FWIW, I hit "millionaire next door" status late last year sometime. Might not have been able to do that, if I had taken on all that debt when I was just 23. But then, who knows how things could have worked out. As it was, I bought a condo in late 1994, got married in July 1995, and then my life went into financial hell. If I had bought that Pathfinder, I probably wouldn't have qualified for the mortgage on the condo, so that might have kept me at home longer. That might have been bad, in and of itself. Or, it may have prompted me to push myself harder and go get a better job. I guess it really could have played out in any number of ways.
BTW, does anybody know what Mr. Stanley's net worth was? Main reason I'm curious, is because that article pointed out what it called "neat irony" that Mr. Stanley met his death in a Corvette, something not normally associated with modesty and living below your means. But if, a guy can afford, say, a Lamborghini, but settles for a 'Vette, isn't that still living below your means?
Also, the guy was born in 1944, so that would make him 70-71 by now...so that 2013 Corvette could still fall into that "millionaire next door" mindset...delay your gratification until you can truly afford it.
