Baen Books sells non-DRM books, in a number of formats. They are the publisher, and they also sell books for a couple of other small publishers:
www.webscription.net. It is mostly science fiction and fantasy.
They are the only major publisher I know of that does not use DRM. Smashwords has non-DRM books, but they are mostly self-published authors with smaller reputations, with a couple of exceptions.