The Kindle works through your Amazon account; the same account you use to buy physical products from Amazon. Each Kindle must be linked to a particular Amazon account, which is then used for payment if and when you buy books on the Kindle itself. You can link your account to multiple Kindles. After buying a book through Amazon, download the book to your Kindle at any time and delete it from that Kindle at any time. Deleting a book from one Kindle does not prevent you from downloading it to, or keeping it on, another Kindle using the same account. Multiple people can use the same account by sharing the ID of the account and the password. This means two people can each read the same book on their own Kindles, even though only one of them has bought it.