FYI, this plugin problems with some ISBN and when that happens it blocks other plugins further down the list such as the official openlibrary.org plugin from providing cover! Note that the official cover download plugins doesn't do this, if they fail the next plug gets to tru.
Unfortunately there doesn't appear to be a way to rearrange the order, it always runs them in alphabetical order.
The error message it gives are: ERROR: Bad cover: The cover is not a valid picture
Example ISBN: 9780345379092
The Amazon Covers plugin fails with what appears to be an internal error, but if it gets the chance the openlibrary.org Covers will provide the cover.
It's hard to tell but I think it probably removes more covers than it adds until this is fixed.