As you might have noticed, I recently added a series of widgets to my blog's sidebar that display all my books. When someone clicks on one of them, it shows several buying options. You can also put an associate ID in the link to get a kickback from any sales through that link.
If you email Fraser Kelton at the company: Adaptive Blue, they'll build a widget for free.
Here's the email if you're interested:
support (at) adaptiveblue (dot) com