Also you can add
The Black Library to the list of DRM-free sites.
I've just purchased books 2 and 3 of the Gaunts Ghosts series and after completing my order I had a "My Downloads" section in my account area that listed them in both epub and mobi. I downloaded the epubs and then to check, I loaded one into Sigil and was able to edit them.
The selection isn't massive, but it is fairly reasonable and they are loading more on all them time.