You have two choices.
1. Do something illegal (crack the DRM). Don't do this. It's unethical. After all, you'll be costing Amazon at least $0.00. Imagine if everyone did this. Multiply that $0.00 by thousands, maybe even millions! All because you had the audacity to expect to make use of something you purchased from them.
OR
2. Accept that you've sold your soul to Amazon and are now locked into their self-reinforcing ebook - reader system with no hope of ever escaping unless you want to re-purchase your library (which will just sell your soul to someone else).
Can you tell I'm cynical about DRM?