To me that sounds like doing all the work upfront for a later, undetermined, and potentially negative return on time. That's opposed to doing the work and realizing the benefit more contemporaneously with that work. Regardless, it's still active work with the return shifted in time. Sounds riskier to me to put all the work upfront without any guarantee of return, especially if that works is particularly intense and active.
> I kind of agree, but at the same time, they only need to be written once and then he can 'sit back'.
Not for the type of books he has written. They are all technical books on specific frameworks (iOS, MongoDB, NodeJS). This reminds me of all the "Office 2001" or "Visual Basic Programming" that I saw in the "Computers" aisle the last time I went to a physical book store. Those books now are a dime a dozen.
Infact, they're written before any income so technically you could say he did a lot of work and can sit back now. Semi-Passive maybe.
The only 'true' passive income types I can think of is just inheriting a huge amount and having it sit earning interest ?