

It may sound complicated if you've never done it before, but it's all very simple and easy. If you plan to use Calibre to store and/or read them, you should download them to your pc then add them to your Calibre library. Once you have an account and a registered device or app, you can browse and download Kindle ebooks from the device or app or you can download to your pc and transfer by usb. Calibre is one of the great tools to manage e-books, most specifically, it can be used to convert your Kindle Book to PDF. If you don't have a Kindle device, download one of these apps depending on which device you plan to use to read your Kindle books: Once you have your account set up, you need to register a Kindle device or Kindle reading app to your account.

For more info about this requirement, see this customer discussion thread on the Amazon forums:

Next, you need to establish a one-click payment method either by associating a credit card or buying an Amazon gift card. To elaborate on pdurrant's reply, even if the book is free you can't download from Amazon without an Amazon account, so if you don't have one you need to open one.
