[jhowell]

(copied from another thread)

Check the Help/About menu to see what version of Kindle for PC you have. Anything before 1.19 doesn't support KFX. If you are using an older version then you probably have updates disabled since Amazon is aggressively pushing the new version to be automatically installed.

If you do have 1.19 or newer there are still some circumstances where books may be present in the older MOBI/KF8 formats supported by calibre.

Any books sent to the app before it was updated to 1.19 will still be in the older formats. You can identify these books because they will be .azw files in the My Kindle Content directory.

Books delivered after the app is updated to 1.19 are placed into subdirectories of My Kindle Content. Some of these will be in KFX format and some will be in the older formats. You can detect KFX because there will be a file with a name beginning with amzn1.drm-voucher.v1 in the same subdirectory.

There are several reasons that a book might still delivered to Kindle for PC 1.19 in an older format.

First, books that do not have Enhanced Typesetting listed on their product page are not delivered in KFX format. (There can be exceptions for fixed layout books.)

Second, it may be a book you purchased a while ago. Amazon locks you in to the version of the book that is current at the time you purchase it. If that version of the book did not support enhanced typesetting, but the product page for the book now shows that it does, you might or might not receive it in KFX format.

Also, KFX format is a moving target. New features are being continuously added to the format and some devices/apps do not support all of the optional features. If a book contains KFX features that a particular device/app does not support then MOBI/KF8 is delivered instead. I have found through testing that Kindle for PC 1.19 doesn't support all of the latest KFX features and so some books are excluded from being delivered in KFX to that app.

Finally, there may be other factors of which I am unaware.