I love how simple it is. No annoying CSS to override to make it fit nicely into my theme, but I do wish it linked the cover image to the Amazon page instead. I wouldn't mind if there was a field that allowed me to manually input a URL to link everything up to, while still using the ISBN to grab the cover image from Google. As it is now I had to code an HTML link to Amazon into the custom message area.
I would also like to see the ability to display more than one book at a time to be displayed (with more good CSS allowing me the option of displaying 2-3 cover images side by side).