>>280853046You couldn't save as an ePub because you didn't have an ePub format saved, only the KFX format saved.
>>280853430Not him, there are two things you need, since you need more effort if you're buying and ripping KFX files from Amazon, it's their proprietary format. If you have them, skip this part.
First, you need the KFX Input plugin,
Optionally, you'll need the KFX output plugin if you have a Kindle and don't want to use Mobi.
Both can be obtained on calibre-ebook's plugins subdomain, or just by Googling them.
I'm also assuming your KFX files have no DRM and you've already handled that.
Once you've installed the plugin, you'll need to convert the KFX file to an ePub first, which can be done by right clicking on the book you wish to convert, clicking the "Convert books" option, and clicking "Convert Individually". Set the output format, on the right, to ePub, and make sure the input format, on the left, says KFX.
Also, messing with ripped comic books is an ordeal and it'll take you a dozen hours of messing around to fully understand it. It could be easier to just buy the book and then pirate it as a CBZ.