All purchases from Amazon follow your Amazon account. If you want to access those purchases, you need to register to that Amazon account. If you go to another account, you ll only see purchases in that account, and purchases in the original Amazon account will not be available.
One Amazon account can actually be shared by several Kindle devices or apps. Here you can find a good discussion on this:
http://budurl.com/KindleContentShare
Source:
http://www.wiki-qa.com/consumer-electronics/handhelds/what-happens-if-i-re-register-my-kindle-to-a-different-account.html