This is the key point. In absolutes there is no such thing as passive income. Anything will take at least 1s of effort, and $0.001 of capital. The spirit of the term, is income that is heavily skewed towards being generated from assets rather than effort.
A new book is the edge case. While making money off IP is generally the perfect example of passive income. Quitting a job to become an author, is not. There is an implied lack of effort and cost in the question. Additionally, and importantly, there needs to be a sense of "free" in there.
The prototypical case:
- [bust your ass at work] >> [get evened out with paycheck] >> ["free!" passive income from savings]
New book case:
- [bust your ass writing] >> [nothing, world still owes you] >> ["free?" passive income from book IP]
Until the "nothing" gets filled in with the equivalent of a salaried wage, I'm not a fan of calling it "passive income".
Would you also consider creating a static "how to" site that generates revenue from ads to not be passive income? I think that's the most basic example of what HN considers passive.
We're just defining words here anyway. I think a core component of what people think of when they think "passive income" is making a disproportional amount of income relative to effort put in. Until those proportions get dis'ed, you're just working at a sub-optimal job.