Update, 4/5/18: I just noticed a post in a MobileRead thread indicating that there are now Calibre plug-ins available for handling KFX. So, there may no longer be any need to use these workarounds.
Things just got a little harder for folks who like to store their Amazon purchases in Calibre. I noticed it myself when I was trying to add a DRM-free title I just bought, Bone Dance by Emma Bull, to my Calibre library. Even though it’s DRM-free, when I dragged and dropped the file from my “My Kindle Content” directory across to Calibre, it couldn’t figure out what to do with the file.
A little detective work turned up this Mobileread forum thread. It turns out that the new version of Kindle for PC, 1.19, no longer uses the AZW ebook format that Calibre knows how to read. Instead, it uses the new KFX ebook format—which nothing else can read or convert right now. The only way to resume being able to convert downloaded Kindle content is to uninstall Kindle for PC, then reinstall Kindle for PC version 1.17, currently accessible via this link (or this one for the OS X version). Once you have, go to the Tools menu, then Options, then General and uncheck the box to install updates automatically—and then just don’t update.
It’s unclear how long Amazon will support these older versions, or that they’ll be available for download. But even if Amazon stops, those who own older Kindle hardware devices will still be able to obtain AZW-format ebooks via the expedient of downloading them to the Kindle then copying them across via USB. Meanwhile, I suppose we can hope that Calibre adds the ability to read KFX files sometime soon.
Update: In a comment, Steve notes you can also download AZW files by going to your “Manage Your Content and Devices” page, then clicking on the “…” button next to the title you want and selecting “Download & transfer via USB.” Even if the Kindle device you choose is a newer model, it still sends you the .AZW file format.
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