Unfortunately, exactly what they invested in wasn't something I was very sensitive to, and I don't remember it.

Generally, they had fairly ordinary incomes, and they invested in things which were considered low-risk at the time. A fair number of them had real estate in the sense of owning car dealerships (used car lots?), with the land under the business being a large part of their wealth.

They disliked spending money. It was common for them to be men whose wives made a full-time job of running the household cheaply. (There was a later book called The Millionaire Woman Next Door.)