Maybe the stylized e-with-red-crescent logo with wings for no DRM, or attached to a lock (or an anchor) for ePub with DRM.
I don't like "plus" for "with DRM" and "minus" for "without DRM;" it implies that DRM-free books are lacking something, rather than that they are more accessible.
The symbol needs to be:
- Scalable to 25x25 pixels or so, preferably 16x16,
- Not dependent on color,
- Functional for people who don't speak/read English (a letter or two is fine; a word is not),
- Elegant enough to appeal to people (ebook providers, ranging from publishing houses to bloggers who offer their archives as ebooks) who don't give a damn about accurate file labels or the DRM issues.