[eschwartz]

Originally Posted by blaenk

Haha, what a coincidence then. Perhaps for me there _is_ a thumbnail because I set the correct ASIN and not a fake one? It did get a thumbnail for the correct book, just that it's a different one from the one I see in Calibre.

Not a coincidence, expected.
A fake ASIN points to nonexistent Amazon product pages, and thus there is no cover to download. The people who configure calibre to embed fake ASINs in order to get rid of the ugly "Personal" banner might end up with no covers at all.
Real ASINs point to existing cover thumbnails, you just prefer the thumbnail you manually embedded in the book.

I'm sorry can you elaborate on this? It sounds to me like I can keep the mobi-asin (so that I could use goodreads) _and_ Calibre should place the thumbnail in system/thumbnails so that the kindle uses that thumbnail and not the one it downloads? Is this a setting somewhere in Calibre? I don't see a system/thumbnails folder. In fact I don't see a system folder at all, I'm pretty sure I used to see one. Perhaps it's because I'm on the latest version of the firmware? I'm using AZW3 and not MOBI, in case that matters.

As long as you use the calibre send-to-device functionality, calibre should sideload a thdumbnail image for any non-PDOC (which means anything with a value, like an ASIN, in the exth 113 field).
For PDOCs, calibre won't bother since the Kindle generates PDOC covers from inside the book.
If you typically sideload books using your file browser, then calibre was never involved, and can't sideload the thumbnail. The Kindle sees a missing thumbnail, and since the book is not a PDOC it attempts to download a cover from the Amazon website.

The missing folder is likely because you are using windows -- Windows will hide a "system" folder in the root of a drive, on the logic that it is a protected system folder.
Tell Windows Explorer to stop hiding protected system folders, or type the folder name into the location bar.

Thanks a ton for your reply eschwartz.

Happy to help!