https://Gumroad.com is a great way to get something like that up and running quickly, too.
An important feature I like is the ability to update the books (due to s/w versions, typos, adding new content, etc) and notify the users regarding the new version.
Another feature for both sites is that customer can pay more than the price of the product. And that does happen often enough to make it a highlight of using their site. But this may probably apply more for technical books than fiction.