very nice, I should take some time to install the latest codebase :)
Regarding the caching issue, when I restructured the bookcover.php module a couple of months ago, I made a shared function that takes care of processing images retrieved from the various APIs. This function takes a "$cache" parameter which controls whether or not the images are stored in the cache, so it's very easy to change which images get cached on a service-by-service basis. Looks like by default, it caches Syndetics, LibraryThing and OpenLibrary but does not cache Google or Amazon images. I'm not sure where these defaults came from originally; I believe I just maintained existing functionality when I refactored… but it's a trivial matter to change the behavior or even make it yet another config.ini setting.