Two things:
1. SF and Fantasy, which I buy as a monthly subscription from Baen (so I wasn't strictly correct in saying that I don't buy commercial eBooks - I just don't buy them from Connect
). Baen's books are RTF files with no DRM, so they're directly readable in Reader - I just bump up the font size a bit first.
2. Out of copyright classics from
http://www.manybooks.net . I absolutely adore authors like Dickens, Dumas, Haggard, etc etc. These form the bulk of my reading and Reader is great for them.