I write fiction, which is a totally different beast from the non-fiction stories you'll find here. Also, I self-publish, so that's even more niche. Still, here's the thousand-yard view of sales.
My first book was sold on Amazon, and podcasted for free on Podiobooks.com. The initial sales were enough to make my car payments for about a half year. Around that time, Amazon made some changes to their ranking, and I dropped in sales by about half. Subsequent books have had the same results (big first few months, but only about a hundred or so per month).
The way you make the most of that is by writing more, and all the normal marketing stuff that you can find online (build an audience). I'm a big fan of Podiobooks.com because, even though it's a bear to do a full audiobook for free, their audience is fairly large and devoted.
As art goes, I was lucky in that I met with a Hugo-award-winning artist (Cheyenne Wright, Girl Genius) who liked my work enough to help me out with covers. I commission each work with him, and he's been a great help to me there.
In case you're interested, here are my books: http://www.amazon.com/Brand-Gamblin/e/B003P4CEFM