Just finished this over the weekend. The Millionaire Next Door is a case study that summarizes the profile of the average American millionaire by examining peoples' financial habits and plopping those people into certain categories based on their saving-to-spending ratios. The types range from prodigious accumulators of wealth to poor accumulators of wealth to everything in-between. The book's main idea is in order to accumulate wealth you should not spend more than you earn. In fact, the opposite behavior comprises the big "secret".
This is the biggest takeaway. Regardless of your actual income, you should try to live WELL BELOW your means if you wish to become wealthy. It's not amount of money that you earn that causes wealth, but the lifestyle decision to maintain a relatively low consumption-to-income ratio that will pay off drastically over time.
The other takeaway was a little disheartening and boiled down to everything depending on a good cohesive inter-generational goal of preserving the family's wealth. What tends to happen to millionaires that live below their means to accumulate wealth is that within a generation they achieve their goal. They have kids and those kids, having every need and just about every want provided, never really learn to live below their means. Compound that with the tendency of wealthy parents to think nothing of providing their adult children with annual allowances, money for a house or the actual house, college tuition, and the like. When the time comes for the kids to inherit the large sum of wealth they'll likely consume and blow through it all within a short period of time unless the parent provided some type of clever trust fund direction that would propagate that wealth into perpetuity no matter what their kids do with their inheritance. Crazy, huh? Not so much. You see this scenario all the time.
I would recommend this book to all would-be prodigal children (and adults) and their parents, or anyone who'd like to see by many an example the difference between trying to achieve wealth by earning a lot of income vs. wisely not spending most of the income you already earn on mindless consumption.