He has written 78 books - some of them make little to no money... or have stopped already. That's a lot of continuous work. The books are about current technologies that are bound to change, if the books are not updated/edited/etc. they are bound to stop making money as well.

Anyways, writing that many book is not passive at any rate.

That's fair, it actually sounds more like a cookie shop where you need to keep baking the cookies for them to sell, even though these cookies can be sold multiple times you still need to keep baking some fresh ones.