My guess would be that leanpub is more "full stack" (formatting, layout, website, etc) whereas gumroad is just a storefront. I use gumroad and like it, but you need a bunch of other tools in your pipeline.

> That depends on where you live I suppose. $40K per year will likely put you in top 1% richest in my country.

Exactly. It only scales up to a point, but that point can still be meanintful.I live in Spain with fairly low cost of living and my (self-published) books have been generating >10k/month profit for several years now, which is several times more than I can spend.

Plus, books are completely zero-maintenance, pure passive income, which is a massive relief compared to any sort of tech product. Would recommend it to anyone who has the interest in writing. (And if you are one of those people, feel free to email me -- I'm happy to help.)

But as OP mentioned, it needs to be a good book. There are a million titles self-published per year and most of them sell approximately zero copies.