Indeed AZK seems to have only been an iOS preview format, so limited in value
https://wiki.mobileread.com/wiki/AZK
Quote:
Overview
AZK is another binary format created by Amazon for reasons that are currently not clear. The Kindle Previewer will convert files to this format if iPhone or iPad is selected.
…
Files in this format can [only?] be downloaded to your iPad or iPhone using iTunes. Unlike other downloaded eBooks that show up in DOCS, this one will show up in Books. Cloud services seem not to recognize this format or offer Kindle as the target.
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Probably only via iTunes, as I found that with direct USB on iPhone no Apple app can see any sideloaded file, but 3 party apps can.
We found the only way to really see KFX on Kindle or Apple Books was to actually publish the epub and buy / download a copy.
The KDP download now simply gives back the apparently identical epub you uploaded (in theory for Send to Kindle). Not sure what happens if you upload docx, but uploading epub to KDP is best and what Customer support told us. Our Apple Books titles go via Smashwords epub, we don't upload to Apple at all.
But surely if you buy a kindle ebook on Amazon on iOS you don't read it in Books but on Kindle for iOS?
Maybe I need to buy an iPad mini. The iPhone 4 can't do much now.