And if my grandmother had wheels she'd be a wagon. This is obfuscating a useful distinction - labour is stuff you do, spending money in the hope of a return is investment. It's like trying to redefine landlordism as labour because you do occasionally have to do some work.
> Moreover, they can be extended by bank investments or Bitcoin investments, extending their "passive"-side
???
The word everyone is looking for in regard to royalties is not passive or active but deferred income. Writing a book is labour, regardless of word games, and it produces a capital asset - intellectual property - that you can either sell all at once or over a period of time.
> the author doesn't need to promote his books
This is rarely true of books. It sounds like he's found an under-served niche. Once upon a time you didn't have to promote apps in the app store, either.