FWIW there are plugins for Calibre which let you easily import DRM-afflicted books, stripping the DRM from them automatically as you go.
I usually run Kindle4PC or Adobe Digital Editions under wine, and then import the download books into Calibre.
A Digital Editions purchase gives you a .acsm file which is essentially an XML blob describing the download. Would be really nice if someone reverse-engineered that and we had a tool which would do it all, rather than needing to run DE under wine and then de-DRM the resulting download. A few beers and a night with mitmproxy should do it.
Would be nice if we had a Kindle implementation which worked the same, of course, but that seems much less likely to happen (and to keep working if it did).