I think I figured out how to set up Rat's Reading to display the preferred iframe-based amazon ads in browsers that read it directly, and img-based ads in atom feeds. Web-based readers like Bloglines or LJ syndicated feeds or the new Google Reader strip out iframes because unscrupulous feeds could steal cookies and what not from the embedding site. I used Wordpress' custom tags and some judicious use of a plug-in that uses the custom tags to generate content. All the examples with that plug-in were for created moods and "currently listening to" type of tags like LJ does. I created a custom tag called "ISBN" that generates the ads.
Not that anyone buys anything from my ads. I wonder why I bother with this minutiae.