I think the feature is part of the Fantastic Fiction plugin. It's been a while (and several versions of Calibre) since I did it and don't quite remember how. But now when I click on Download Cover on the Edit Metadata screen, Fantastic Fiction is one of the sources it grabs from.
I think Fiction DB has a plug in, too, but their covers are ofter too small. I do like their site for missing blurbs.
|