well, for backup purposes, they do keep them on their site until you delete them (or at least, that's the way it's supposed to work).
If you download the epub versions, then you're downloading ther ACSM file which in turn will download the book within ADE. once you have that epub book in ADE, you can move it to any medium you like, however, you will only be able to open it within ADE or other device that uses ADE DRM with the username/password that you downloaded it with.
of course, to get around this, you could always DRM it, but that I can't help you with
