So you have bought a nice e-ink based e-book reader, and you need to quench your thirst for books. How do you get them? Free e-books are, well, free and without encryption; paid e-books, on the other hand, can be either encrypted or unencrypted.
For the sake of simplicity, I will assume that your reader is able to read ePub and PDF files and supports the Adobe Digital Editions DRM for both formats. Most readers can however be reprogrammed to support Mobi files, however, but I personally suggest to stick with ePub.