I have this shortcut answer as I use it A LOT when merging and bundling and making custom covers based off the existing covers.
If just wanting copies of covers that show in Calibre:
Save to Disk...
- in Preferences go to Save to Disk
- check off first box for "save cover separately".
Then pick the books and Save to disk (better to only save one format such as epub for speed). Then all cover jpegs are wherever you saved. You can easily delete the book files. The covers also name the same way the book file was set to name in the Save Template part in the preferences rather than as just "cover.jpg".
If you are asking about books that DO NOT SHOW COVERS when browsing the calibre library book details and only give the default book icon, you can set the covers from the ebook file (assuming it contains one) by using a batch Edit Metadata feature. It doesn't appear to work one at a time which is odd.
Add a blank, "junk" or "test" book if needed if you need to grab an extra book to do this feature or use one that already exists correctly.
- Select Multiple Books
- Edit Metadata
- "Set from ebook file(s)" is near bottom right of window.
- IF you are going to export this cover after doing this, you MUST use the Polish Plug-in to update the cover in the book so that this is never a problem again. I use this plug-in all the time for many reasons. It's great and easy.
If the book doesn't have a cover.jpg inside, then you will have to add it either by "download metadata" in the Edit Metadata feature OR get your own from the internet or you own files via Browse.
A note: you can drag and drop a jpeg from the internet to the book cover when in Edit Metadata mode. I've read you can also drag it onto the book details window or onto the details pane, but I've haven't been able to get that to work.
THEN, you can also use Save to Disk on these to get the covers as separate files as describe above.
This should do it. It is FAST and easy!