Well, selling an e-book you've written is still profiting from your own labour; it's not really generating income whether you work or not, despite the fact that you don't have to actually mind the store very much. You're just stretching out the return on work you've already done. And in practise, you will have to do work updating and promoting your book if you don't want sales to decline to near-zero.

"Generating income whether you work or not," to me, implies things where you're collecting an economic rent rather than actually producing anything. If you're a leftist, a libertarian, or certain kinds of paleoconservative, you probably won't want to do that. I'm just suggesting that people should carefully consider their own values when considering ways of making money.

But perhaps I'm reading too much into the topic, as I haven't read the book in question.