LibraryThing just opened access to around a million cover images uploaded by LT users; you can get to them just by signing up for an API key and grabbing them from a special URL.
Tim Spalding makes some disclaimers: Amazon has more and probably better cover images, and they impose no maximum limit. However Amazon requires you to backlink to them from each image. LT has no such limit, and in fact encourages you to locally store images in order to reduce load on LT's servers.
If you installed the Millennium Module for Drupal, you can take advantage of LT's cover images like this: