I found this to be a fascinating and most helpful book. It was so motivating I'm already working on three new streams of income, and about to start a fourth. Forget net worth! Cash flow is much more important, particularly if you're retired. Only one slight criticism of the book. It's a bit dated, but those few parts make little difference to its overall value. If you're currently struggling with how you're going to survive after you retire, try Allen's approach. It will open your eyes.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.