It’s outdated as far as referencing information contained within. It’s just detailed enough to make you feel like your getting some good information but in reality since the links don’t work your really getting nothing except some information that you then have to find detailed answers elsewhere, to bad they turned off their website with that supposed information. I also emailed the company for links for these detailed answers that they left me wondering and they never replied. I would pass on this book unless you want to get a general idea of some things to look at doing but they are not worth the read since they failed to uphold their website that had more information.
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.