Hi Bookworm,
I'm not sure if this will help you, but I found that I need to "Polish" a book, and imbed the cover into the epub file, then convert it to mobi again. When I transfer this new file, or an AZW3 with the cover imbedded, the covers show up on my fire.
I don't change PDOC to EBOK, at least not conciously, and all my books transferred from calibre show up as books not docs - I think I just had to set things up (maybe in calibre preferences - Sending Books to Devices) so that the books were going to the books folder - I don't have my fire with me today so I can't check. I don't use whispersync though, so things may be different with that.
Anyway, I think imbedding the cover is the key thing.
Good luck!
Liz
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