Also consider sources of eBooks that are not DRM infested. For example, all eBooks from Baen publishing are DRM free and available in a wide variety of formats, including ePub. (
www.baenebooks.com. Baen publishes fantasy and science fiction, with a good deal of it being military sf. They also have a LOT of books available completely free at the
Baen Free Library. A great way to check out an author and see if you want more from her/him.