From Wikipedia:
> Passive income also know as "Unearned Income" is income that can be earned automatically with minimal labor to earn or maintain. It can be a great way to help one generate extra cash flow, whether he or she is running a side hustle or just trying to get a little extra dough each month.
According to the above definition, the author earns $$$ each year after he had written each book and he doesn't need to do a lot of stuff to earn or maintain (people just buy on Amazon and upvote his books, e.g.). So it is passive income. ("minimal labor to earn" =>>> the author doesn't need to create ads to promote his books etc., he just write them once... and this writting is equivalent to "low labor", see below).
What you want to highlist is rather: SINCE he has worked a lot to earn $10k with 78 books (not only one book - which you wrote in your comment) written over a period of several years, THEN: is it actually worth in terms of $$$?
Given the fact he probably didn't spend lots of $$$ to write and publish his books & ebooks (but he didn't give this piece of information), and given the fact that writting a book is far less stressing than working for someone (in fact, in addition to his job, it made him relaxing), I would say YES. The time he has spent on each book is controlled since in his article, he emphasizes on the fact that he writtes 100+ pages books, not more: so even considering "time is money", the time with no income required to write his books may have been refunded with the sells quite quick.
*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.)
Moreover, since he already have a fulltime job, I bet he uses his books income to invest as banks investments or, perhaps, on Bitcoin, etc (since his fulltime job decreases the risks for him to have a big debt e.g., in the case of his eventual investments were not worth). Such investments would extend the "worthly"-side of his books activity, alongside with very controlled risks thanks to his fulltime job.
PS: I think 2 or 3 points above have medium probability to be wrong, because the author didn't communicate about them, leading to some speculation/hypothesises by myself. So don't take this comment too serious. However I think it's consistent thoughts.