How can I share books with another Kindle user?

There is a section in the Kindle device 'Settings' screen that handles device account registration.as long as the device has a live wireless connection, you can change account registration any time you want, you just have to know the email address and password of the account that your trying to register to, upon confirmation of account registration, your device is given full access to download and buy any content that you want from the "new" account. Then when you done with downloading whatever content your downloading from the "new" account, then you can de-register/register back to your "old" account, and then read any of the books that are currently on your device, no matter what account they were downloaded from.