Diane (nb Gypsy Rover AU) and I did some experimenting with our Amazon Kindle book readers the week before. We wanted to try "sharing" books we'd bought.
First we de-registered Diane's kindle from her Amazon account and added it to my Amazon account. When she turned the Kindle back on she was able to download all the books from my account. Excellent!
We plugged Diane's Kindle into her laptop and copied all her books into a folder on the desktop. She couldn't remember the password for their Amazon account and as the time in the UK was about 3am or something we couldn't ask Ray. So by copying the books onto the desktop and them plugging my Kindle into the laptop we could then copy the books onto my Kindle. It all seemed to work fine.
Then we encountered some problems. When I linked my Kindle back to my own account, the books belonging to Diane remained on the Kindle but when I tried to open one it came up with a message saying you can only read it when the Kindle is linked to the account that bought the book.
Diane's Kindle was not linked to any account - due to the problem of not remembering the password to put it back on her own Amazon account. But, when she opened one of my books she was able to read it! She decided to leave her Kindle un-registered until she'd read all the books from my account.
As it has turned out that was a good idea. Today I de-registered my Kindle to see if I could then open one of Diane's books. No luck. It seems it's a lucky quirk that Diane has been able to!
All the same it's great to be able to share books but you need to leave the other person's Kindle registered to your Amazon account for them to be able to read your books. You should only do this with someone you trust as they would be able to buy books that would be charged to your credit card during the time their Kindle is registered to your account. Either you, or they, can de-register it at any time.