Search for boox. They come in all sizes. But not in FLOSS philosophy
It's not libre, but I would recommend Kobo as the "least worst" alternative.
It's very easy to sideload anything, the devices are nice to use and the built-in store has some DRM-free books available (though it's mostly DRM stuff).
For more technically-minded people there are also various projects giving Kobos totally libre unofficial firmware.
@rgggn Tolino uses OSS only, I am not sure about the F though. Also there might be an exception for the stuff opening DRM-infested pdfs.
But it runs on android, so you can root it (german article, but nice syntax so it should be machine-translatable with a reasonable quality).
If it's in budget I'd recommend the vision 4 HD model for its support for redshift-like behaviour. (Also I can't judge about the rest since I only ever had a vision 4 HD in my hands.)
@rgggn Try Onyx
@rgggn i don't think duokan is foss tho
BQ Cervantes runs a libre firmware based on Debian Wheezy. As to DRM-free bookstores, that's a tough one, aside from indie books and NoStarchPress.