For Win8.1 users, make sure you download the Kindle For PC app -
the Win7/Vista version. That one will set up the My Kindle Library in your Documents folder, just like it always was, and you'll be able to access your books if you need to run them through Calibre. Otherwise if you click on the Win8 version it only lets you download that app for the Metro screens. If you do that, then you don't have access to the whole book files - they are hidden as fragment files in:
C:\Users\(Your Name)\AppData\Local\Packages\AMZNMobileLLC.Kindlef orWindows8_....\LocalState\cache
I have no idea how you would re-assemble them if you needed to convert them into an epub for another of your ereaders.
You have to unhide the "Hidden Files" in order to see the AppData Folders.
I'd also recommend archiving that Kindle installation file for future use in case it disappears once MS stops supporting Win7.