Does anyone know what books Kiyoksaki is referring to?

[Andrew McCartin]

In Rich Dad Poor Dad, Kiyosaki writes, “On every one of my investments, there must be an upside, or something for free - like land, a house, stock shares, or an idea. There are books devoted entirely to this subject, so I will not talk about it here.”

Does anyone have any suggestions for books that fit this description? Would love to read more on this concept!