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I have a couple of Personal Document books that do show cover thumbnails on my PW1. I have never bothered to try to figure out why, as I don't use Cover view on that device. calibre never touched them. I think there may be a 'magic' cover resolution that will get thumbnail image generated, at least that used to be the case on Fire.
Thumbnails show for all of the books on my Fire HD6 (and ios, android) once they're downloaded (except Fire only shows them in Collections, Carousel, and task switcher, not in Docs view).
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