[johnnyb]

I just received an email from Amazon telling me that a new ‘and improved’ Kindle for Mac app will be coming in September and that the current app will stop working in October and will be discontinued. They promise reading and library management improvements for the new app.
They explain in detail how to transfer collections from the old app to the new one (start them side by side for the transfer to initiate) but there's no new app in the Mac App Store yet (where it should be ‘starting September’).

My guess is that they will finally bring the iOS app to Mac (like they did with Prime video), which should bring a few welcome improvements in terms of library management and should also be a snappier experience overall.