The EFF has posted a new guide to maintaining your rights while using ebooks. I don't like the previous versions, but this one is pretty useful. It does a good job of clarifying the related issues.
Below we offer a checklist that can help guide your inquiry, as well as an extended explanation of why the answers to these questions matter. Not surprisingly, some of the issues overlap. For example, Digital Rights Management, or "DRM," matters not only because of the limits it places on users, but because of its impact on innovation and competition. Yet by separating out the various issues, we hope to spur a more rigorous consideration of the various digital book offerings.