Welcome to Mobileread
Amazons strategy was good (for a time) - Sell ereader & sell books.
However as you look around this site you'll see many other ereaders.
Each of them have some good or bad points.
The best ereader though is the one that suits you. Later you may decide that you want something different.
Your LIBRE will not work with Amazons ebooks without cracking the copy protection and then converting it to the format that you want.
Amazon will, I think, want to sell ebooks to everyone. But not yet.
Look around Mobileread for a while. Learn.
Your LIBRE will be fine, and there are a great many places to find books other than at Amazon.
Start at: Reading Recommendations
or for more info about the LIBRE at: Ectaco jetBook