[Synamon]

If anyone with a Kindle is downloading to K4PC only to get the books on their computer and into calibre, there's a better way.

Select the 'Transfer via Computer' option when you one click purchase a book or 'Download & transfer via USB' from the Actions button in your Kindle Library (Manage Your Kindle page). You'll get a popup and you can choose the Kindle you are downloading the content for, then another popup asking what folder on your computer you want to send the file to (I created a folder called 'Amazon downloads'). The ebook file that downloads won't have the number gobbledygook, it will have the book title which makes it much easier to manage.

Even better, you can set up calibre to automatically add the books from the folder that you download the files to (Preferences > Add Books > (tab) Automatic Adding). Note: calibre will delete the file from the folder after it auto adds it and you need a recent version, 0.8.44 or so.