Labor to write his book is low labor, since he writes books <=100 pages, he enjoys that, and it is far less stressing than working for someone (employer/customer). In fact it makes him feeling happy and, perhaps, relaxed. However, the author would be welcome to clearly indicate weither or not this required labor is actually "low one" or not, because he didn't communicate about that.
Maintaining his books: the author doesn't communicate about that but maintenance operations on books seem low labor too, far less than code maintenance.
Those books incomes, I see them as passive(writting a book once, over a period of several months probably, then eventually getting money each month is as worth as investments, except that investments are done in few seconds with some money needed to be brought - for books written: no money is needed to be brought, but it takes months contrary to few seconds to be done). Moreover, they can be extended by bank investments or Bitcoin investments, extending their "passive"-side.
> PS: Sadly writing can pay off as much as giving you nothing (money wise).
Exact. But the author writes no books > 100 pages and he has even written a sentence about this.
PS: if we define "labor" by "spent money", then writting and publishing a book (and ebook) is also low labor since very few $$$ are required to be bring by the author, like computer science projects.
> Note that: bank investment need lots of money to be done. Writting books, no. Bank investment need a few seconds to be done. Writting books, several months. I think they are equivalent and, thus, are both ways to earn passive incomes.
("equivalence" from a money point of view and almost from a labor point of view, if we consider that bank investments's initial brought money is only due to labor, not inheritance, etc.)