$1000 a month on book sales.

I was surprised since I'm a software guy I didn't write the book to make money, I just did it for something to do.

Although my software business makes much more money, I was surprised at how truly "zero maintenance" book sales are. My software I'm constantly fixing, tweaking and improving (which I enjoy). The book is just "out there" and is priced at $30 per copy. I sell > 1 per day.

The book: http://www.growthhackinghandbook.com