Any recommendations where to buy DRM-free epubs?

I recently got an XTEink X4 and installed the open source Crosspoint Reader firmware (I really enjoy the small form factor)

I tried beam-shop.de and they have a decent, all DRM-free catalogue.

I set it up for a friend yesterday and we exchanged a Euro back and forth so he got the 5 € welcome bonus. Seems to be a good incentive to get people on board. Hope my main bank implements it this year.