Hi Kovid,
Could I ask you to consider adding support to Calibre for Amazon's AZK format for iOS Kindle applications? As I'm sure you know, iOS Kindle apps don't support KF8 (AZW3) files; instead, Amazon has an apparently closely-related format called "AZK" which is the equivalent for these apps. AZK files can be created by the Kindle Previewer tool and side-loaded to iOS Kindle Apps.
I realise that this request doesn't stand much chance of being fulfilled, because it's such a specialist format, but nothing ventured, nothing gained...
Thanks
.