Originally Posted by kiwidude
Maybe Kovid has changed things in the last few weeks but one of the limitations I found for my Goodreads covers download is that you can *only* download covers for books with an ISBN. It is hard-baked into a number of places in the Calibre code that if the book has no ISBN then you are out of luck. I reported this as a ticket here
and Kovid said it would be addressed as part of the new API.
Well it definitely didn't seem to want to download covers without an ISBN, though the Overdrive plugin doesn't use ISBN. However it was retrieving ISBN data beforehand from google/isbndb before attempting cover retrieval. The problem was that instead of picking the first/best ISBN from ISBNDB/Google it seemed to be searching for all of the covers at once, and was also over-writing the title in every case.
The difference I'm seeing for covers vs. metadata providers is it's not checking the validity of the ISBN before cover download, but for metadata it will only proceed to get metadata after confirming the ISBN is in Google's or ISBNDB's databases.
Now that I think about I have a hunch that this is because three interfaces are sharing the same code - bulk download, download cover button, and 'Fetch Metadata' button - the fetch metadata button checks each ISBN to see if a cover exists so it can display that info to the user - I suspect it's that bit that doesn't play well with the overdrive plugin.