If you happen to like science fiction and fantasy,
Baen Books sells without DRM (much to the detriment of my wallet). O'Reilly does the same for technical books, although while they're recent books by famous (in their fields) authors, somehow I don't think they're what you're looking for.
Also, check your favorite authors' websites; they may have their backlist, additional stories, or even whole recent books, available.
I won't buy books with DRM for philosophical reasons (the vote of my wallet may be tiny, but it's all the vote I have) and this has led me to discover all sorts of interesting authors I might never have read otherwise. And, of course, there are always the classics -- Project Gutenberg, here on MobileRead, Feedbooks, Manybooks, and Munsey's being popular sources.