I was recently sent an email by Christina Bobul, the Eco-Friendly Media Evangelist of Green Books LLC. The idea that they are promoting is that switching from paper books to electronic books will help protect the environment. I agree that this might be true, but only if the books are interoperable with the widest range of computing hardware and software such that people aren't ever forced to put far more toxic computer hardware prematurely into landfill.
I looked at their website and could not find any information about the file format they are using for their digital books. It should be clear that this business could not claim to be eco-friendly if they were using "DRM" or other proprietary file formats.